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December 12, 1985 ^ypgp,S11 flaux

M^ ' WS

Annual Technical Report for Grant No. NAGW-661

Covering the period from December 1, 1984 to November 30, 1985

Measurements of H02 Chemical Kinetics with a New Detection Method

Submitted by:

Long C. Lee, Masako Suto, and E. R. Manzanares

Department of Electrical & Computer Engineering

San Diego State University

San Diego, CA 92182

Tel. (619) 265-3701

1

Prepared for:

National Aeronautics and Space Administration

NASA Headquarters

Washington, D. C. 20546

Attention: Dr. R. F. Hampson

Upper Atmosphere Research Program, Code EE

17u417) t!EASUbLMcN'1^ Cr HCZ

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05C 17
I. Introduction .............................................. 3

II. Research Accomplished .....................................3

A. Experimental Apparatus ................................ 3

B. Production and Detection of H02 ....................... 5

C. Reaction Rate Constant of H02 + 03 ....................5

D. Photoexcitation Process of C12 ........................9

III. Cumulative Publications and Presentations ................12

IV. Appendix .................................................13

"Quantitative VUV Spectroscopy of C12"

2
This report describes the results obtained in the period

from December 1, 1984 to November 30, 1985 for the research

program currently supported by NASA under Grant No. MAGM-661

basic. In this research program, H02 was detected by the OH(A-X)

photofragment from dissociative excitation of H02 at 147 nm.

This de_ection method was applied to measure the reaction rate

constant of H02 * 03. This reaction rate constant is needed for

the understanding of stratospheric chemistry.

Since C12 was used in the flow system, photoexcitation of

C12 may produce fluorescence to interfere with our measurements.

Thus, the photoexcitation process of C12 in the vacuum

ultraviolet region was also examined in this research period

using synchrotron radiation as a light source. The research

results are summarized below.

II. RESEARCH ACCOMPLISHED

A. Experimental Apparatus

A discharge-flow-tube apparatus has been constructed in the

current funding period. The schematic diagram for the apparatus

is shown in Fig. 1. The flow tube consists of a large tube of

4.8 cm ID and 60 cm long, and a movable infector of 1 cm ID and

70 cm long. (The large tube of 4.8 ID was later changed to 2.3

cm to check the of '4--is o:A the reaction rate

measurement). Inside the infector there is a small teflon tube

of 2 mm ID. 03 was produced by a high vcltage a.c. discharge of

02 and stored in two silica gel traps attached to the flow tube.

3
V

C d

^ O

p iJ

H O cau

O C

vn a

O

O

a

M

0

ar

^ 0

.1

eo x

C

O

Hu

H

W

T

F ^I

v

.a

u

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w

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?a tea

wcn

U U

H

O

C.^

 o

a

a+

v

cc u ^

G

d

oD

H •^

W G^

0.

X

W

N

O

-4

cn 5 1 1 y U

00

a

F4 
The 03 concentrations were monitored at two positions before and

after the flow tube. H02 in a gas cell was detected by a Xe

resonance light at 147 nm. The gas cell was a six-way cross of

3 11 OD which was made of stainless steel.

H. Production and Detection of H0!;o

H02 was produced by microwave discharge of a trace of C12 in

He, which then reacted with CH30H and 02 by

Cl + CH30H -► CH2OH + HC1 (1)

CH2OH + 02 4 H02 + CH2O (2)

The measurements of gas pressures and the control of ga: slaw

rates has been described in detail in the Semiannual Report.

H02 was detected by the OH(A-X) emission produced from

photodissociative excitation of H02 by a Xe resonance light at

147 nm,

H02 + b y (147 nm) - OH*(A) + 0 (3)

OH*(A) OH(X) + b y (306-320 nm) (4)

The attenuation of the light source and the quenching of

OH*(A) by the gases in the gas cell were carefully examined,

which were described in detail in the Semiannual Report.

C. Reaction Rate Constant of H07 + On

The reaction rate constant of H02 + 03 has been measured by

Zahniser and Howard [ J. Chem. Phys. 73. 1620 ( 1980)] to be

2x10- 15 cm3/s,

MO2 + 03 4 OH + 202 t^

OH will react with 03 to form H02 again,

OH + 03 -► H02 + 02 ( f

with a reaction rate constant of 6.5x10- 14 cm3 / s. In order 1

measure the rate constant of reaction (S), the OH radicals neo

5
to be scavenged. C2F3C1 and C3Hg were used as the OH scavengers

in our experiments,

OH + C2F3C1 products k7 = 6x10- 12 cm3 /0 (7)

OH + C3Hg products ke a 1.1x10 -12 cm3 /s (a)

For 1033 varied in the 30-100 mtorr range in our

experiments, 100 mtorr of C2F3C1 or 2CO mtorr of C3Hg will

effectively eliminate OH in the system. Thus, the effect of OH

on the H02 + 03 reaction rate measurement is minimized.

At each 1031, the psuedo-first-order-decay rate, K, of H02

was measured by the relative H02 concentration in the gas cell as

a function of the H02 + 03 reaction time by changing the H02 flow

distance in the flow tube. The K values measured at various 1031

are shown in Fig. 2, where (200-250) mtorr of C3H8 was used as

the OH scavenger. The reaction rate constant determined from the

plot is (2.1 ± 0.5)x10- 15 cm3 /8. This result is consistent with

the value of Zahniser and Howard of 2.0x10 -15 cm3 /8.

We also measured the H02 + 03 reaction rate constant using

C2F3C1 as the OH scavenger. This molecule has been used by

Zahniser and Howard in their H02 + 03 kinetics measurements. The

psuedo-first-order-decay rates of H02 as a function of 1031 are

shown in Fig. 3, where about 120 mtorr of C2F3C1 was used as the

OH scavenger. The reaction rate constant determined from the

plot is (1.5 ± 0. 4) x10 -15 cm3 /s. In this measurement, we observed

that C2F3C1 produced fluorescet;.;a in C.e UV region when it was

excited by the 147 nm light. Although the fluorescence intensity

was relatively small when a narrow band filter (310 ± 10 nm) was

used to isolate the emission band, the fluorescence may somewhat

6
0

O

O

cl,

O

OO

GO

cn

O

0

F I

Fco -:r C14 C)

Fig. 2

7
'41 (ti G

Fig. 3

0

0

0

N H

M

U

U1

1

O

XM1

1

.!L

N

O

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E

O

N

.-1

u

U

M

WN

U

U

-ri

3

O M

u'1 O

0

(I_s)x

8
disturb the observed signal. The fluorescence intensity from 

C2F3C1 has been corrected in the decay rate measurement.

In order to check the effect of walls on the measured

reaction rate constant, we changed the flow tube size from 4.8

cm ID to 2.3 cm ID. The psuedo-first-order-decay rates of H02

measured with this small flow tube as a function of 1031 are

shown in Fig. 4, where C2F3C1 was used as the OH scavenger. The

reaction rate constant determined from the plot is

(1.7 ± 0.2)x10 -15 cm3 /8. This result will be checked again using

C3H8 as the OH scavenger.

From the above results, the rate constant of the H02 + 03

reaction (averaged over the data points) is (1.8 ± 0.3)x10'15

cm3 /8. We will take more data to reduce the experimental

uncertainty, and then report the results in a scientific ,journal.

D. Photoexcitation Process of Cl

C12 was used in the flow system to produce Cl atom that

finally made H02. We observed that C12 has a weak ultraviolet

fluorescence when excited at 147 nm. It is interesting to study

the fluorescence process, because it is useful to assess the

interference of the C12 fluorescence to our kinetics

measurements. In addition, the photodissociation process of C12

is also of interest in the study of the upper atmospheric

photochemistry. The photodissociation of C12 by vacuum

ultraviolet radiation in the upper atmosphere is a source of Cl

atom. Based on these needs, we studied the photoexcitation

process of C12 using synchrotron radiation in the 105-200 nm

region. The results are summarized in a paper entitled

"Quantitative VUV Spectroscopy of C12 0 which is attached as an

dP
4

I

21

0 60

(mtorr)

a III
F

Appendix. This paper has been submitted to the Journal of

Chemical Physics for publication.

The absorption of C12 in the VUV region is dominated by

strong discrete structures. 147 nm is at the tail of the

absorption band of the 1 1 E' Rydberg state. Both absorption and

fluorescence cross secticns of C12 at 147 nm are quite small,

thus, the fluorescence does not impose a serious interference to

our kinetics measurements. The minor effect caused by the

f fluorescence from the photoexcitation of C12 was corrected in the

data shown in Figs. 2-4.

11
III, CUMULATIVE PUBLICATIONS AND PRESENTATIONS

Oir! preliminary works have been reported in scientific

community as listed below:

1. M. Suto and L. C. Lee, "Phototragment Emission from MO2 and

its Application to the H02 Kinetics Study", presented at the

Informal Atmospheric Kinetics Meeting, Jet Propulsion

Laboratory, Pasadena, CA, Jan. 18, 1985.

2. M. Suto, " Study of the H02 + 03 Reaction by Detecting H02

from Photofragmen'. Emission", Seminar presented to the Naval

Research Laboratory, Washington D. C., June 14, 1985.

3. L. C. Lee and M. Suto, " Photoexcitation Process of C12 in 

VUV", presented at the 1985 SRC Users Group Meeting,

Madison, Wisconsin, Oct. 21-22, 1485.

4. L. C. Lee, M. Suto and K. Y. Tang, " Quantitative VUV

Spectroscopy of C12", submitted to J. Chem. Phys. (1995).

12
IV. APPENDIX

"Quantitative VUV Spectroscopy of C12"

13
Quantitative VUV Spectroscopy of C12

L. C. Lee and Masako Suto

Department of Electrical and Computer Engineering

San Di,

San

Western

9555

San

ago State University

Diego, CA 92182

and

K. Y. Tang

Research Cooperation

Distribution Ave.

Diego, CA 92121

ABSTRACT

The photoebsorption and fluorescence cross sections of C12

were measured in the 105-145 nm region using synchrotron

radiation as a light source. The oscillator strengths for the

mayor absorption bands are calculated from the measured

absorption cross sections. The measured oscillator strengths for

the 23 7u and 211Iu F X 1 "" transitions agree quite well with the

theoretical values. The absorption spectrum was analyzed in

accord with the excited electronic states calculated by

Peyerimhoff and Buenker (1.981). The Rydberg series converging to

the first and second ionization potentials wers classified. The

vibrational levels of the 2 1Eu ionic state were determined up to

v' = 15 from the fluorescence excitation spectrum. It was

observed that a band with peak at 108.3 nm produces VUV

fluorescence, but it produces UV fluorescence only by a

collisional excitation process.

1
I. INTRODUCTION

The electronic transitions of chlorine molecule in the

vacuum ultraviolet (VUV) region have been subject to extensive

investigatiens. 1-4

Lee and Walshl

observed the absorption

spectrum in the 107-210 nm region with a 1-m vacuum spectrograph.

The absorption bands for wavelengths longer than 130 nm were

assigned to possible electronic states in accord with molecularorbital theory. However, the absorption bands at wavelengths

shorter than 130 nm were so complicated that they were not

analyzed. Iczkowski et al. 2 revisited the absorption spectrum in

the 110-135 nm region in order to clarify the possible

impurity (HC1) contamination of the C12 spectrum observed by Lee

and Walsh i . A Rydberg series coverging to the first ionization

potential at 11.47 eV was found.2

The rotationally resolved absorption spectrum was observed

and analyzed in the 133-145 nm region by Douglas 3 using a 10.7 m

vacuum spectrograph. An ionic state, 1 1 E`, was determined from

U

the analysis. The absorption spectrum in the 107-130 nm region

has been taken by Douglas but it was not analyzed because of 

complexity. Recently, Moeller et al. 4 studied the absorption

spectrum of C12 in the 120-140 nm region using synchrotron

radiation. Fluorescence from the 1 1 Z* state was observed and

u

dispersed. The absorption soect-im wnsg int.eT'DI'eted with the

potential curves calculated by Peyerimhoff and Buenker 5 . The

nature of the electronic transition of the absorption spectrum

in the 130-145 nm region is now well understood, 1-5 although the

transitions in the 105-130 nm region are still not clear.

2
The absolute absorption and fluorescence cross sections of

C12 in the VUV region are reported in this paper. These data are

useful for understanding the nature of electronic states in the

short wavelength region. The quantitative data are also needed

in several application areas. For example, C12 has been used in

our laboratory for the study of H02 kinetics. 6 The fluorescence

cross section of C12 is needed for examining the effect of C12

fluorescence on the kinetics measurement. C12 is also generally

used in the chemical etching of semiconductor materials and in

the rare gas-excimer laser. The quantitative photoexcitation

data of C12 are useful for understanding the role its plays in

various environments. In addition to these pracitcal

applications, the oscillator strengths determined from the

measured absorption cross sections are useful for comparison with

the results of ab initio calculations.?

II. EXPERIMENTAL

The experimental set-up for the current measurement has been

described in a previous paper. 8 In brief, synchrotron radiation

produced from the electron storage ring at the University of

Wisconein was dispersed by a 1-m Seya vacuum monochromator. The

optical path of the absorption cell was 39.2 cm. The VUV light

source was converted into UV light by sodium salicylate coated 

outside a LiF window and then detected by a photomultiplier 1-utb

(PMT). The fluorescence was monitored by a PMT (EMI 9558G)B) in

the UV-visible region and a Polar blind PMT (EMI, CsI cathode) in

the VUV region. The UV and VUV fluorescences were observed in

directions orthogonal to the light beam. The UV fluorescence

3
cross section was calibrated by comparing the C12 fluorescence

intensity with the OH(A-X) fluorescence from Nhotodissociation of

H2O, for which the fluorescence cross section is known- 8 The

fluorescence quantum yield is then determined from the ratio of

fluorescence cross section to absorption cross section. The

absolute cross section for the production of VUV fluorescence was

calibrated by comparing the observed C12 fluorescence intensity

with the CO resonance fluorescence intensity, for which the CO

fluorescence cross section is assumed to be equal to the CO

absorption cross section.

Nature C12 was supplied by Matheson with a stated purity of

better than 99.9%, which was used in this experiment as 

delivered. The gas pressure was measured by a MKS Baratron

capacitance manometer. The light source intensity, the gas

pressure, and the fluorescence intensities were simultaneously

monitored and analyzed by an IBM microcomputer.

III. RESULTS AND DISCUSSION

A. Photoabsor ption Cross Section

The photoabsorption cross section of `=12 in the 105-145 nm

region is shown in Fig. 1 for a monochromator resolution set at

0.1 nm. The cross section was derived from the light attenuation

under the condition that the absorbance increases linearly with

increasing C12 pressure. The pressure range used in the

absorption measurement depends on the absorption cross section,

that is, the higher the absorption cross section, the smaller is

the pressure range.

As shown in Fig. 1, the absorpticn spectrum is dominated by

4
sharp discrete structures. Thus, the measured cross section

depends on the monochromator resolution. The apparent absorption

cross section given in Fig. 1 represents the average value over

the 0. 1 nm bandwidth. The uncertainty is estimated to be ± IOX

of the given value. The wavelengths given by the high resolution

spectra of Douglas3 and Moeller et al . 4 were used as the

wavelength reference in this experiment.

In contrast to the absorption cross section, the oscillator

strength does not depend on the resolution. The oscillator

strength can be determined 9 from the measured average absorption

cross section by

f = 11. 3 !d(a )daA 2

where Q (X ) is the absorption cross section in unit of Mb

(10 -18 cm2 ) and a is the wavelength in unit of nm. The

oscillator strengths for the strong absorption bands are listed

in Table I. The experimental uncertainty is estimated to be

±10% of the given value.

The oscillator strengths for the 21:7u and 23 ^' u F X 1 F g

transitons have been recently calculated by Grein et a1. 7 whose

values are 0.0699 and 0.0084, respectively. The sum of the

measured oscillator strengths for the 2 1 7u k =0-3) -f-- X I Y* is

0.0635 as shown in Table I and Fig. 1. This measured value

agrees very well with the calculated one. The oscillator

strength fc: the 1';; u +.v'=0 ; t- X 1 F $ transition determined from

the data shown in Fig. 1 is 0 . 017. This value is larger than the

calculated value by a factor of 2. However, depending on the

energy difference between the 23pu and 21nu states the calculated

value7 could be adjusted to 0.0122. This higher value agrees

5
reasonably well with our measurement. In considering both the

experimental and theoretical uncertainties, the agreement is

quite good.

The absorption bands are classified into Rydberg series, as

shown in Fig. 1. The wavelengths, the wavenumbers, and the peak

absorption cross sections for these Rydberg states are listed in

Tables II-IV. The justification of the Rydberg state assignment

is discussed below.

B. Rvdbera States

The electronic configurations of C12 have been studied by

Lee and Walsh l , for which the ground state, X l l* , is given by

---(6g3p)2tffu3p)4(71g3p)4

The potential curves of C12 have been calculated by

Peyerimhoff and Buenkers . The singlet states relevant to the

interpretation of the observed strong transitions are shown in

Fig. 2. Although the vertical excitation energies may need to be

adjusted7 , these potential curves are still good to serve the

purpose of present discussion. The potential curves for most

excited states are complicated by the perturbation of the ionic

states such that they result. in double wells.

The 1 1 E u ( Irg 4p n) state is an ionic state which

fluoresces. The vibrational levels of this ionic state were

studied in great detail by Moeller et al. 4 , and the rotational

spectra were analyzed by Douglas 3 . The fluorescence quantum

yield for this state is close to 1 (see Discussion in the next

section).

The 21 n u state results from the 7F g - 4pa Rydberg

transition. 5 The effective quantum number of this Rydberg state 

6
is n* a 2.46 ( a quantum defect of d = 1.54 for n=4) , where the

ionization potentia110 of 92,590 cm- 1 (11.48 eV) was used for the

calculation of n a . Using this quantum defect, the Rydberg states

with n = 5 and 6 are found as listed in Table 71 and also

indicated in Fig. 1. The spin-orbital splitting for the first

C12' state, X2?Igl/2,3/2, is same as its vibrational frequency,

_ thus, the photoelectron spectrum shows a single vibrational

progression. 11,12 The characteristics of the Rydberg states are

usually the same as its converging ion state, that is the spinorbital splitting for the 2 l H u and 3Hu states are also similar to

the vibrational frequency as shown in Fig. 1.

The vibrational levels of the 2 1 7' state were assigned by

Moeller et al. 4 The vibrational levels are now extended to v'=15

z

using the fluorescence excitation spectrum observed in the

current experiment (see discussion in the next section). The

2 1 Eu state results from the 7T g-> 4p T7 Rydberg transition s ( which

may mix with the ag -► au transition) with an effective quantum

number of n* = 2.75 ( a quantum defect of d =1.25 for n=4) .

Using this quantum defect, the n = 5 and 6 Rydberg states are

found as listed in Table III and also indicated in Fig. 1.

The absorption spectrum in the 105-115 nm region is

complicated, and the transitions were not assigned before.

According to the potential curves shown in Fig. 2, the absorption

in this wavelength region likely corresponds to the 3 1 1 Iu-; X1E9

transiton. The calculated vertical excitation energy for this

transition? is 90501 cm -1 (110.50 nm). The 3 1 H Ustate results
from the (Yu-* 4s Rydberg transiton 5, which converges to the

second C12' state, A2ni. The vibrational levels of this ion

state were observed to be a chaotic distribution. 13 The

vibrational levels were classified into five vibrational

s'

progressions, that is, system I (a), (b) and (c) and system II

(d) and (e). Since the 311T u state is the Rydberg state

converging to the C12*(A) state, the vibrational spacings of

these two states should be similar. This expectation is indeed

true. The absorption structures in the 105-115 nm region are

arranged into system I (a), (b) and (c) and system II (d) and

(e) as shown in Fig. 1 in corresponding to the vibrational

progressions of the C12* (A) state. The wavelengths,

vibrational frequencies, spacings, and peak cores sections for

system I (a) and (b) are listed in Table IV. Both the chaotic

distributions observed in the Rydberg and ion states indicate

t that the chaotic vibrational distribution occurs in nature. The

other poesibility 13 that the chaotic vibrational distribution of

the ion state is due to perturbation seems unlikely. The

oscillator strengths for the system I (c) and system II (d) and

(e) are relatively weak when compared with system I (a) and (b).

The weaker systems may be due to the triplet state, 3 3rr u, similar

to the case of 2 3 TIu mixed with 217u•

Taking the ionization potential ll,12,14 of the C1 2 +(A) state

at onset as 13.96 eV, the effective quantum number for the first

vibrational level of system I(a) is n* = 2.10 (the quantum defect

6= 1.90 for n = 4). Considering the quantum defect of 1.54 for

the p-electron, this quantum defect of d = 1.90 for the selectron is reasonable. This further supports the assignment of

8

ftit'—, : _ _
the 31 % Rydberg state.

C. Fluorescence Cross Sections

The cross sections for the production of UV and VUV

fluorescence measured at a monochromator resolution of 0.2 nm are 

shown in Fig. 3. The absorption cross section measured at the

same resolution is also shown in Fig. 3 for comparison. The peak

absorption cross section in Fig. 3(a) is generally smaller than

that in Fig. 1, because the monochromator resolution used for

Fig. 3 ( a) is lower.

The VUV fluorescence was detected by a solar blind PMT

whose response was in the 115-180 nm region, and the UV

fluorescence by a PMT ( EMI 95580B) in the 180-800 region (see

the Experimental Section for the absolute calibration2. The

uncertainty for the UV fluorescence cross section is estimated to

be ± 30% of the given value, but for the VUV fluorescence the 

uncertainty can be a factor of 2.

The fluorescence from the 1 1 EU ionic state has been observed

by several investigators,4,15.16 although the absolute 

fluorescence cross section was not reported before. The

fluorescence excitation spectrum to the 130-140 nm region has

been studied by Moeller et al. 4

They also dispersed the

fluorescence spectrum which covers from the excitation wavelength

to 208 nm4 . In our experiment, when an optical filter

(transmitting wavelengths longer than 300 nm) was placed in front

of PMT, the fluorescence disappeared. This confirms that the

fluorescence spectrum is limited in the UV and VUV regions. The

VUV fluorescence likely results from the 1 1 Eu 4 X 1 Eg* transition,

9
and the UV fluroesconce from the 11E 11E9

transition.

The VUV and UV fluorescence excitation spectra for the 11 EU

state are generally the same an the absorption spectrum an shown

in Fig. 3, except for the band at 136.2 nm whore the VUV

fluorescence in relatively morn intense than the UV fluorescence.

The croon section for the VUV fluorescence in larger than the UV

fluorescence by a factor of about 5. At each wavelength in the

135-150 nm region, the sum of the VUV and UV fluorescence cross

sections is about equal to the absorption cross section, 

ff indicating that the fluorescence quantum yield is nearly equal to

1. This result shows that the l l"L U+ state is not predissociative.

At low gas pressure, both the UV and VUV fluorescence

intensities of the 1 1 1 + state increase with increasing gas

pressure as shown in Fig. 4(a), where the excitation wavelength

is 135.2 nm. The fluorescence intensity does not increase

further at high gas pressure. This saturation effect is due to

the pre-absorption of light source intensity by the gas between

the entrance and the PMT view region. The absorbance, which is

defined as ln(I O /1) for the light source intensities with (I) and

without (I o ) C1 2 in the gas cell, is also shown in Fig. 4(a) for

comparison. The saturation of the absorbance at high gas

pressure is due to total absorption of some rotational lines by

the gas. For the absorption cross section measurement the

pressure is limited to the region that the absorbance is linearly

dependent on the gas pressure an mentioned before.

The fluorescence from the second ionic state, 21 E U

+ ,, in not

well studied. We observe that this state fluoresces up to v'z15

an shown in Fig. 3(b). The wavelengths of the observed

10
vibrational levels are listed in Table II. This result

demonstrates that the fluorescence excitation spectrum provides

useful information to identify electronic states, which are

otherwise difficult to be identified from the absorption spectrum

because of overlapping of several electronic transitions. The

fluorescence quantum yield for the 2 1 1: U* state is generally less

than 1, indicating that this state is partially dissociative.

This state may be heavily perturbed by the triplet states that do

not fluoresce.

At wavelengths shorter than 109.7 nm, a VUV fluorescence

band appears as shown in Fig. 3(b). At low gas pressure, the

VUV fluorescence intensity increases linearly with gas pressure

as shown in Fig. 4(b). At high gas pressure, the VUV

fluorescence intensity is saturated due to pre-absorption. The

VUV fluorescence is saturated at the gas pressure lower than that

for the absorbance. This is caused by the fact that the VUV

fluorescence is absorbed by the C12 gas before it reaches the

window of the solar blind PMT. Because of such difficulty as

well as the spectral response being not corrected, the

uncertainty for the absolute VUV fluorescence crone section in

this excitation wavelength region is qutie high (a factor of 2-

3). Nevertheless, the fluorescence quantum yield for this

excitation band le quite large. The excitation energy of phis

absorption band is higher than the ionization potential, thus, it

is a no-called "superexcited state". The fluorescence yield for

a • superexcited state" is usually small. The high fluorescence

yield observed in this excitation band presents an interesting

li

fAt
case.

When the C12 pressure increases, fluorescence in the UV

region appears. The UV fluorescence intensity depends on the gas 

pressure quadratically an shown in Fig. 4(b), where the

excitatic- wavelength is 108.25 nm. This result indicates that

the UV fluorescence is not primarly produced by photoexcitation

but by collisional-induced excitation. The UV fluorescence may

result from energy transfer from excited species to C12, where

the excited species are produced by photoexcitation. The cross

section for the production of UV fluorescence by primary

photoexcitation process is practically zero. The dasb f d curve

shown in Fig. 3(c) is only an excitation spectrum, which does not

represent the absolute fluorescence cross section for the primary

photoexcitation process. It is noted that the UV fluorescence

has a significant intensity at few mtorr. This indicates that

the excited state has a sufficient long lifetime which could be

f

4 longer "-. An 20 us as estimated from gas collision kinetics.

This lifetime is much longer than the lifetime of the VUV

radiative state that is expected to be in the no region. This

result indicates that the UV and VUV fluorescence are produced

}

from different states.

The nature of the fluorescence band in the 106-110 nm region

is not clear and it deserves to be studied more. The excitation

spectra for both UV and VUV fluorescence at pressures 2.0, 4.9

and 11.1 mtorr are shown in Fig. 5. The intensity for the VUV

fluorescence increases linearly with C12, but the UV fluorescence

increases quadratically for the entire excitation band. The

light source intensity in this wavelength region is nearly

12
13

constant, and it is not corrected in Fig. S. It is noted that

the excitation spectrum for the UV fluorescence is different from

the VUV fluorescence, indicating that they are produced from

different excited states as expected from the estimation of the

different lifetimes. Similar structures appear at both the UV

and VUV excitation spectra as well -,r repeatly shown in the

absorption spectrum.

The VUV fluorescence is not produced from the excited atomic

chlorine, because the excitation photon energy is not enough to

produce excited chlorine atoms. For instance, the dissociation

energy 17 of C12 is 2.479 eV, the wavelength threshold for

producing the C1( 2P- 2P3/2) emission cf 134.7 nm is 106.1 nm.

This threshold wavelength is shorter than the threshold of the

fluorescence band that starts at 110 nm.

IV. CONCLUDING RLMARK

The photoabsorption and fluorescence crass sections of C12

in the VUV region are measured using synchrotron radiation as e

light source. The structures shown in the absorption spectrum

are classifed into Rydberg series. The structures shown in the

photoabsorption spectrum are generally observed in the thresholdelectron-impact excitation spectrum 18 and the electron v.iwrgy

loss spectrum. 19 The reAnlu*ir-n Orr the current photoabsorption

spectrum is much higher tha,• , the electron spectra, 18.19 thus, the

structures are much better resolved and assigned.

The Fluorescence yield for the 1 1 £* state is very high

(close to 1). Thus, this fluorescence system can be used to
develop techniques for a remote sensing of C12 in various

environments, such as plasma etching vessel. In other case, the

fluorescence cross section of C12 at 147 nm is so small that the

existence of C12 in the flow tube does not affect the H02

kinetics measurement. The fluorescence excitation spectrum is

also a useful tool for spectroscopic: study. As an example, the

vibrational levels of the 2 11* ionic state are observed up to

v'=15 with the aid of fluorescence excitation spectrum. It is

interesting that the absorption band in the 106-110 nm region has

a large quantum yield to produce VUV fluorescence, but the yield

for the UV fluorescence is essentially zero. The UV

fluorescence is produced by a collisonal excitation process. The 

nature of this excitation process is not understood and it will

be interesting to pursue further.

ACKNOWLEDGEMENT

f This research program in partly in cooperation with Dr. T.

J. Chuang at IBM Laboratory, San Jose. We wish to thank Dr. C.

S. Lee, Dr. E. R. Manzanares, Dr. J. B Nee, and Dr. W. C. Wang in

our

R

laboratory for useful discussion. The authors are grateful

to the staff of the Synchrotron Radiation Center of the

University of Wisconsin. The SRC facility is supported by the

NSF under Grant No. DMR-77-23888. This vorV is nuppc,r+Pd by the

NSF under Gant No. CBT-8518555 and the NASA under Grant No.

NAGW-661.

14
REFERENCES

1. J. Lee and A. D. Walsh, Trans. Faraday Sac. = 1281 (1959).

2. R. P. Iczkovski, J. L. Margrave and J. W. Green, J. Chen.

Phys. ,a, 1261 (1960) .

3. A. E. Douglas, "an. J. Phys. 599, 835 (1981) .

4. T. Moeller, B. :-)rdan, P. Gurtler, G. Zimmerer, D. Haaks, J.

LeCalve, M. Castex, Chem. Phys. 76. 295 (1983) ; idib. ,

Spectral Li ne hapes. Vol. 2, Ed. K. Burnett ( Walter de

Gruyter, Berlin, 1983), P. 597.

5. S. D. Peyerimhoff and R. J. Buenker, Chem. Phys. 57. 279

(1981).

6. W. C. Wang, M. Suto, and L. C. Lee, J. Chem. Phys. 81. 3122

(1984).

7. F. Grein, S. D. Peyerimhoff, and R. J. Buenker, Can. J. Phys.

62. 1928 (1984) .

8. L. C. Lee, J. Chem. Phys. 72. 4334 (1980) .

9. L. C. Lee and J. A. Guest, J. Phys. B: At. Mo 1. Phys. 14.

:3415 (1981).

10. K. Watanabe, J, Chem. Phys. 26. 542 (1957) .

11. A. W. Potts and W. C. Price, Trans. Faraday Sac. t37, 1242

(1971) .

15
12. A. B. Cornford, D. C. Frost, C. A. McDowell, J. L. Rsgle,

and I. A. Stenhouse, J. Chem. Phys. ZA, 2651 (1971) .

13. F. P. Huberman, J. Mol . Spectros. ?& 29 (1966) .

14. T. A. Carlson, M. 0. Krause, F. A. Grimm, and T. A. Whitley,

J. Chem. Phys. 7_$, 638 (1983) .

15. Y. V. Rao and P. Venketeaverlu, J. Mol. Spectrosc. 9 173

(1962) .

16. A. E. Douglas and A. R. Hoy, Can. J. Phys. 53. 1965 (1975) .

17. K. P. Huber and G. Herzberg, Constants of Diatomic

Molecules. (Van Hostrand, New York, 1979).

18. J. Jureta, S. Cvejanovic, M. Kurepa, and D. Cvejanovic, Z.

Phys. A. 304, 143 (1982).

19. D. Spence, R. H. Huebner, H. Tanaka, M. A. Dillon, and R. G.

Wang, J. Chem. Phys. 80. 2989 (1984) .

16
I

Table I

Integration of absorption cross section over wavelength, IodX, in units of

Mb.nm and oscillator strength, f, in units of 10- 3 forithe major absorption j

bands.

wavel ength (nm) 10(X)dX(Mb•nm) f(10-3)

111.50-111.77 21.7 19.7

112.58-112.85 22.9 20.4

113.17-113.39 19.0 16.7

113.66-114.01 29.4 25.6

114.12-114.47 22.4 19.4

116.96-117.15 10.1 8.3

117.82-117.98 9.7 7.9

124.74-125.00 9.1 6.6

126.36-126.63 12.7 9.0

127.84-128.38 31.6 21.8

130.84-131.14 5.9 3.9

132.00-132.19 19.6 12.7

133.11-133.30 36.2 23.1

134.22-134.47 37.8 23.7

135.01-135.95 83.5 51.5

135.95-136.49 43.0 26.2

17
Table II 

Effective quantum number, n* , wavelength, X(nm), wavenumber, v(cm -1 ), vibrational

spacing, Av(cm 1 ), and peak absorption cross section, cSP ( Mb),for the'$g -► np a

Rydberg Series.

n n* v' a(nm) y(cm 1) w(cm-1) qt Mb) (

4 2.46 0 134.33 74,443 301.5

(2 1IIu ) 1 133.22 75,064 621 315.6

2 132.10 75,700 636 157.6

3 131.04 76,313 613 39.0

4 130.14 76,840 527 

5 129.22 77,387 547

5 3.46 0 119.87 83,424 35.3

1 119.02 84,019 595 42.9

2 118.18 84,617 598

6 4.41 0 115.02 86,941

1 114.40 87,413 472

00 108.00 92,590

18 
Table III

Spectroscopic data for the Ttg -► npn Rydberg Series

n n* v' a(nm) y(cm-1 Oy(cm l ) at.(Mb)

4 2.75 0 128.01 78,119 133.9

(21 E

u) 1 126.45 79,083 964 84.3

2 124.89 80,070 987 58.6

3 123.66 80,867 797 28.2

4 122.33 81,746 879

5 121.17 82,529 783

6a 120.18 83,209 680

7 118.94 84,076 867

8 117.92 84,803 727

9 116.81 85,609 806

10 116.02 86,192 583

11 115.18 86,821 629

12 114.28 8 7, 504 683

13 113.37 88,207 703

14 112.59 88,818 611

15 111.81 89,437 619

5 3.75 0 117.92 84,803 84.9

1 117.08 85,412 609 94.9

2 116.32 85,970 558 54.7

3 115.50 86,580 610

6 4.68 0 114.18 37,581

Oa 108.00 92,590

a The wavelengths for the vibrational levels from v' -6 to 15 Were determined from

the fluorescence excitation spectra.

19
Table IV

Spectroscopic data for the iru - 49 (3 1IIu) Rydberg Transition

Vibrational Series a(nm) y(cm 1 ) ev(cm 1 y^

I a 113.91 87,789 143.1.

112.75 88,692 903

130.3

111.65 89,566 874

101.5

110.72 90,318 752

109.70 91,158 840

108.94 91,794 636

108.20 92,421 627

107.46 93,058 637

106.72 93,703 645

b 113.28 88,277 122.6

112.06 89,238 961

110.98 90,106 868

110.02 90,893 787

109.25 91,533 640

108.49 92,174 641

107.75 92,807 633

107.03 93,432 625

20
FIGURE CAPTIONS

Fig. 1 Photoabsorption cross section of C12 in the 105-145 nm

region measured with 0.1 nm resolution. The wavelength

positions for the Rydberg transitions are indicated.

Fig. 2 The potential curves of the singlet states relevant to

the discussion in this paper. These curves are adopted

prom Ref. 5.

Fig. 3 (a) absorption, (b) VUV fluorescence, and (c) UV

fluorescence cross sections measured with 0.2 nm

resolution. The vibrational levels of the 2 1 E u are

indicated in (b). The dashed curve in (c) is an

excitation spectrum which does not represent absolute

value.

Fig. 4 The pressure dependence of the absorbance, VUV and UV

fluorescence intensities measured at (a) 135.2Q nm and

(b) 108.25 nm.

Fig. 5 Dependence of excitation spectra of (a) VUV and (b) UV

fluorescence on C12 pressures of (1) 2.0, (2) 4.9, and

(3) 11.1 mtorr.

21
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