2P174
Anisotropic interaction between iodomethanes and He*(23S) metastable atoms
and autoionization processes studied bycollision-energy-resolved Penning ionization electron spectroscopy
in the energy range of 20 - 350 meV(Department of Chemistry, Graduate School of Science, Tohoku University)○ Andriy Borodin, Takuya Horio, Masakazu Yamazaki, Naoki Kishimoto, and Koichi Ohno
Penning ionization of iodomethanes (CHxI4-x) upon collision with metastable He*(23S)
atoms was studied by collision-energy/electron-energy resolved two-dimensional Penningionization electron spectroscopy (2D-PIES) in the collisional energy range from 20 to 350
meV.
The initial guess for the eclectronic structure of iodomethane and diiodomethane wasmade by the ab initio calculation. The main difference between band positions observed by
UPS and calculated by the OVGF method is the splitting of non-bonded MOs caused byspin-orbit interaction and Jahn-Teller effect,1 those are known, in particular, for 5p band of I
(5p1/2 - 5p3/2) and can also be observed for related MOs in many compounds containing Iodine.
The PIES spectra of iodomethane and diiodomethane are similar to the UPS spectra except for
the relative band intensities. In PIES, some additional structures caused by the interaction
with metastable He* can be noticed.
The Figure 1 shows the HeI UPS and He*(23S) PIES spectra of iodomethane. The
calculated positions of valence bands (byOVGF) are presented below the spectra. Theelectron density maps related to that are also
shown in the figure. The MOs 9e and 8e aredegenerated in OVGF, so the OVGF
positions and electron density maps cannot
be divided between them. Unlike UPS, astrong additional structure (marked by S*)
stretching from Ee ~ 0.5 eV to Ee ~ 4.5 eV
can be noticed in PIES. A peak at Ee ~ 4.2
eV as well as several small irregular peaks
in region of 1–4 eV reproducing themselves
Figure 1. He*(23S) PIES and HeI UPS of iodomethne
form one measurement to other belong tothis structure. A similar additional broad band was already observed by PIES for HI.2 In thatcase the enhanced peak can be assigned to ionic states of I+ produced via autoionisationwithin the potential energy surface of a transient HI-–He+ ionic pair. In case ofbromomethanes (CHxBr4-x) a similar process can be observed in PIES: the autoionization ofthe Br** Rydberg atom produced at the dissociation of molecular target caused a similar S*band.3 On that way, the neutral dissociation followed by autoionization processes can also

explain well the appearance of the S* - band for iodomethane.
As can be seen in the Figure 2, the collision-energy-dependences of the partial ionization
cross sections (CEDPICS) for all MOs of iodomethanehave negative slopes indicated an attractive character ofinteraction with He*(23S) in low collision energy range.
In the full collision energy range, these dependences arenegative for MOs 1 – 3, with slopes of -0.30 – -0.38. The
slopes 4 and 5 should rather be related to theautoionization band S*, which is tightly surrounded by
the satellite bands. And the shape for the slope of band S*,
in its turn, is similar to that of band 3, which is a σ bandof C–I bond, which may be connected to theautoionisation from σ*CI excitation states (LUMO, next toLUMO) caused the appearance of band S*.
The anisotropy of the interaction potential between
3CH3I and metastable He*(23S) is shown in Figure 3 by the Figure 2. CEDPICS for He(2 S)+CH3I
model potential calculation with a Li(22S)-atom. The curve denoted by (□) for the He* (Li22S) approaching towards the I atom from directions which are perpendicular to the C–I bondare most attractive (the well depth of 318 meV); while thecurve (●) for direction along the C–I bond has a weakly
negative or rather neutral character (the interaction
potential of -20 – -30 meV). The curve (▼) for thedirection towards methyl group has a repulsive character.
The behavior of the interaction potential is wellcorrelated with the slopes of CEDPICS in the range of thecollision energy of 150 – 300 meV.
The diiodomethane (CH2I2) shows a similar behavior
in interaction with the metastable He*(23S): The bands
indicating the aoutoionization can be observed; theionization from all MOs has negative CEDPICS by
attractive interactions; the interaction potential has
similar values of the well depth in regions around the I Figure 3. Interaction potential curves
for Li(22S) + CH3I
atoms and close to the methyl group.
Thus the collision-energy-resolved Penning ionization electron spectroscopy inconnection with the ab initio calculations gives us a good means to study the collisional
ionization dynamics in detail such as the dissociation following autoionization observed in thepresent study.
References:(1) Ehland, J. H. D. Photoelectron Spectroscopy: An introduction to Ultraviolet Photoelectron
Spectroscopy in Gas Phase (Butterworth, London, 1974).
(2) Yencha, A. J.; Ruf, M. W. and Hotop. H. Z. Phys. D. 1994, 29, 163.
(3) Kishimoto, N.; Matsumura, E.; Ohno, K.; Deleuze, M. S. J. Chem. Phys. 2004, 121, 3074.