PHYSICAL REVIEW LETTERSPRL 95, 246102 (2005)

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9 DECEMBER 2005

Excitation of Molecular Vibrational Modes with Inelastic Scanning Tunneling Microscopy
Processes: Examination through Action Spectra of cis-2-Butene on Pd(110)
Yasuyuki Sainoo,1,* Yousoo Kim,1,†,‡ Toshiro Okawa,2 Tadahiro Komeda,3,6
Hidemi Shigekawa,4,6 and Maki Kawai1,5,6,†,x
1

RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Department of Physics, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan
3
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan
4
Institute of Applied physics, 21th Century COE, NANO project, University of Tsukuba, 305-8573, Japan
5
Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8651, Japan
6
CREST, Japan Science and Technology Corporation (JST), Toshima-ku, Tokyo 171-0031, Japan(Received 8 March 2005; published 5 December 2005)
2

Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce
vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on
Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes
than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action
spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection
rules in terms of resonance tunneling.
DOI: 10.1103/PhysRevLett.95.246102

PACS numbers: 68.35.Fx, 63.22.+m, 68.37.Ef, 82.65.+r

The excitation of molecular vibration by means of theinelastically tunneled electrons from the tip of a scanningtunneling microscope (STM) can lead to various dynamical processes at surfaces [1,2]. In addition, inelastic electron tunneling spectroscopy with the STM (STM-IETS) [3]is now applicable to the vibrational spectroscopy of theindividual molecules. The vibrational spectrum of a single
molecule provides useful information not only for thechemical identification of the molecule [4–6], but also
for investigating how molecular vibration can couple
with the relevant dynamical processes [7,8].
However, many experimental results [1,3–8] haveshown that not all the vibrational modes are observed in
STM-IET spectra, which indicates that the establishmentof proper selection rules is necessary. In order to establish
the selection rules, it is of crucial importance to clarify
experimental observation of the molecular vibration induced by inelastically tunneled electrons. STM-IETS detects the vibrational modes of a single molecule by measuring the total conductance change resulting from theinelastic electron tunneling. However, the total conductance change is not always detectable, since the elastic
component can be adversely reduced giving rise to a decrease in IET signals for some vibrational modes [9]. The
response of vibrationally mediated molecular motion toapplied bias voltage, namely, an ‘‘action spectrum,’’ canreveal vibrational modes that are not visible in STM-IETS,
because the molecular motion is induced via only inelastic
tunneling. Thus, the action spectrum would be a candidatefor detecting which vibrational mode is actually excited
and associated with molecular motions. Here, we show theusefulness of action spectroscopy as an alternative vibrational spectroscopic method for the STM-IETS throughthe study of vibrationally mediated reversible motion of a
0031-9007=05=95(24)=246102(4)$23.00

cis-2-butene molecule among four equivalent adsorption
orientations on Pd(110) at 4.8 K. The STM-IET spectrum
shows only two peaks corresponding to the metal-carbon
stretch mode, M-C, and the CH stretch mode in the CH3
group, CH3 . However, the action spectrum reveals that
not only M-C and CH3  but also the C-C stretch
mode, C-C, and the bending mode in CH3 , CH3 ,are actually excited via inelastic tunneling leading to the
motions.
All experiments were performed using a commerciallyavailable low-temperature STM (LT-STM, OmicronGmbH) equipped in an ultrahigh-vacuum chamber (<3 
109 Pa) [8,10]. The Pd(110) surface was cleaned bycycles of Ar ion sputtering and annealing cycles, andexposed to cis-2-butene molecules below 50 K. All images
and spectroscopic data were acquired at 4.8 K; experimental details for spectroscopy have been reported elsewhere[8,11,12].
A cis-2-butene molecule adsorbed on Pd(110) appears asa gourd shape in the STM image with a large bright region(head) and a small less bright region in Fig. 1(a). There are
four equivalent orientations, labeled CUR , CUL , CDR , and
CDL . An isolated cis-2-butene is -bonded to off-centeredposition of the Pd atom, where the molecule is slightly
shifted towards the hollow site [11–13]. The adsorption
geometry is proposed as shown in Fig. 1(b).
The procedure for inducing molecular motion is described previously [12]. In brief, after imaging the target
molecule at initial orientation, the STM tip is positioned
over the center of the head part of the molecule. Then, thebias voltage is increased to a certain value and the tunneling current is recorded as a function of time. A rescanningof the same area shows that the molecule has changed its
orientations one to the other. The repeated sequences show

246102-1© 2005 The American Physical Society

CDL

T

(b)
CUR

CDL

CUL

CDR

T

CUL

CDR

[110]

T

T

current [nA]

(c)

I1

3
2

LB motion

3
1

1

[001]

I2

0

0.52

1.0

2.0

0.56 0.60
Time [sec]

0.64

I3
I4

HB motion

FIG. 1. (a) STM images of four equivalent orientations of acis-2-butene molecule labeled CUR , CDR , CUL , and CDL , which
are obtained by moving the molecule by injecting tunneling
electrons. The suffixes UR, UL, DR, and DL correspond to therelative location of the head part, i.e., up-right, up-left, downright, and down-left, with respect to the center of the molecule in
the STM images, respectively. A trans-2-butene molecule (labeled T) was coadsorbed as a marker. The superimposed gridindicates the position of Pd atoms, which is determined from the
STM image obtained with a molecular tip. The additional four
dotted lines forming a rhombus correspond to the molecular longaxes at each orientation. An asterisk represents the center of thelarge bright region of the molecule in each orientation, where thetunneling electrons were injected. (b) Proposed structure of cis2-butene on Pd(110) at four equivalent adsorption orientations.
(c) Current changes measured at sample bias voltage of 170 mV
with the tip fixed over the center of the head of a cis-2-butene
molecule. The inset shows the wide time range of the same data.

that the trace of tunneling current reflects residence time of
the molecule at a certain orientation. Figure 1(c) shows atypical change in tunneling current while the sample bias
voltage is kept constant at 170 mV.
The motions of cis-2-butene can be categorized into two
types [12]; they are extracted from the difference in current
changes and simplified by defining the pairs of (CUR , CUL )and of (CDR , CDL ); the pairs are indicated by arrows in
Figs. 1(a) and 1(b). CUR and CUL (CDR and CDL ) are mirror
 while CUR
images with respect to a plane parallel to 110,and CDR (CUL and CDL ) are also mirror images with respect
to a plane parallel to [001]. The motion with small changes
in current, such as either between I1 and I2 or between I3
and I4 in Fig. 1(c), corresponds to the change between twoorientations with the mirror plane parallel to [110]. This
motion is named ‘‘low barrier (LB)’’ motion. The othermotion is named ‘‘high barrier (HB)’’ motion with larger

current change between (I1 , I2 ) and (I3 , I4 ) in Fig. 1(c),corresponding to the orientation change between two pairs
for LB motion. The potential barrier for HB motion ishigher than that of LB motion, which is clearly seen inaction spectra.
Figure 2 shows the action spectra of cis-2-butene, where
the motion yield (number of molecular motions per injected electron) is indicated as a function of applied sample
bias voltage at a chosen tunneling current. There are clear
thresholds for both LB and HB motions of C4 H8 in theupper spectra of Fig. 2(a). For LB motion, the motion yield
markedly increased at 37 mV followed by a slight increase at 115 mV. For HB motion, on the other hand,clear increases were observed at 115 and 360 mV. A
comparison between C4 H8 and fully deuterated cis-2butene (C4 D8 ) helps to assign the active vibrational modes
to the above-mentioned motion. In the action spectrum of
C4 D8 , the increases of the motion yield for LB motion wereobserved at 31 mV and at 95 mV. For HB motion, theincreases of the motion yield appeared at 110 mV and at
270 mV. The assignment for those modes is described
below with combing to STM-IETS, which gives us thevibrational signature of an individual molecule.
Figure 3 shows the STM-IET spectra of C4 H8 [Fig. 3(a)]and C4 D8 [Fig. 3(b)]. The significant features appear at
(a)
10-8
10-10
10

C4H8

-12

LB

HB

LB

HB

10-8
10-10
10-12

0

100

(b)
M o t i o ns / e l e c t r o n

CUR

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PHYSICAL REVIEW LETTERSPRL 95, 246102 (2005)

C4D8

200
300
Sample bias (mV)

400

LB

C4H8
C4D8
40

100
160
Sample bias (mV)

FIG. 2. (a) Action spectra for described motions both ofC4 H8 (upper) and of C4 D8 (lower). Data were taken under fixedtunneling current of 3 nA for C4 H8 and of 2 nA for C4 D8 .
(b) Magnification of the action spectrum for low barrier motion
at around the threshold energy. Slight increase in the yield wasobserved around 115 mV for C4 H8 and 95 mV for C4 D8 , asindicated by arrows, respectively.

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PHYSICAL REVIEW LETTERSPRL 95, 246102 (2005)
0

-200

-400
358
-3

3
36
0

0

C4H8
0

200
400
Sample Bias (mV)(b)
d 2 I/dV2 (nA/V2)

d 2 I/dV2 (nA/V2)

(a)

0

-200

32

-400

268

3

-3

0

0

C4D8
0

200
400
Sample Bias (mV)

FIG. 3. STM-IETS spectra taken over (a) C4 H8 and (b) C4 D8 .
Solid lines and dashed lines represent vibrational spectra of thepositive and negative bias range, respectively. The background
spectrum taken over bare Pd(110) surface has been subtracted
from each spectrum. All spectra were obtained at a tunneling gap
set point of 0.25 nA and 20 mV, and Vrms 15 mV ac modulation
at 797 Hz. Each spectrum is the average of 16 bias voltage
ramps.

358 mV for C4 H8 and at 268 mV for C4 D8 . Identicalsignals were observed in the STM-IET spectra of trans-2butene on Pd(110), and they were assigned to the vibrational peaks for CH3  and CD3 , respectively [8]. An
additional feature with a very low intensity at about 36(32) mV for C4 H8 (C4 D8 ) was observed repeatedly for different sets of molecule and tip. No vibrational feature otherthan the two peaks was observable in the STM-IET spectra.
The high-resolution electron energy loss spectroscopy(HREELS) spectrum of cis-2-butene on Pd(110) [13]presents four groups of vibrational energies observed indifferent regions: (1) metal-carbon stretching mode at 20 –40 meV; (2) CH bending and C-C stretching modes atapproximately 100 meV; (3) C——C stretching mode,
C—
C,
at
160–180
meV;
and
(4)
CH stretching mode—
at approximately 360 meV.
Comparing with the HREELS spectrum, it turns out thatboth the first threshold in the action spectrum for LB
motion of C4 H8 (C4 D8 ) observed at 37 (31) mV and thesmall peak observed in STM-IET spectrum at 36 (32) mV
correspond to the vibrational energy of M-C. Thus, weconclude that the excitation of M-C directly couples
with LB motion. For region (2), while the lower thresholdfor HB motion was found at 110 mV both for C4 H8 andfor C4 D8 [Fig. 2(a)], a clear isotopic shift was observed
from 115 (C4 H8 ) to 95 mV (C4 D8 ) for LB motion in
Fig. 2(b). The latter case can be explained with the isotope
shift of CH3  and CD3 . On the other hand, C-C
should be expected to show a weak isotope shift. Thus,
these two modes overlap in energy for C4 H8 but are
separated for C4 D8 , and both modes contribute to inducing
molecular motion. For region (4), it is obvious that CH3 
corresponds to both vibrational signals, observed at 358
(268) mV in the STM-IETS spectrum and at 360
270 mV in the action spectrum of HB motion of
C4 H8 (C4 D8 ). However, C——C in region (3) was invisible in both the STM-IETS and the action spectrum. A

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similar behavior was observed in the hopping motion ofC2 H4 on Pd(110), where no response was observed in the—C stretching mode region [14].
C—
From the above-mentioned analysis of the action spectra, it is clear that the vibrational modes M-C, CH3 ,C-C and CH3  are excited via the inelastic electron
tunneling. Since the M-C mode is directly related to the
motion of the molecule, the excitation of all the higher vibrational modes can induce the same motion in their relaxation processes. The activation barrier for the motion of
the molecule discussed here is pretty small, and the resultof motion rate measurement as a function of applied current indicates that the excitation of two quanta of C-M issufficient to overcome the barrier [15]. However, although
the stretching mode and the bending mode of sp3 CH in
CH3 groups were clearly observed in the action spectra,—C bond were not observed.
those for sp2 CH at the C—
Note that —CH and —CH for sp2 CH appear inregions of different energies from those of sp3 CH’s, typically at 380 meV and at 80 meV, respectively, in HREELS
spectrum [13,16]. In order to explain this issue, we discuss
the mechanism of the excitation of molecular vibration
modes in STM via the inelastic electron tunneling.
Within an STM junction, electrons from the tip firstencounter the molecular orbital at the lowest unoccupied
molecular orbital (LUMO) level forming a temporary
negative-ion state, inducing slight distortion of the molecule [17]. Since the lifetime of the temporary negative-ion
state should be extremely short, the distorted molecule in
the electronic ground state remains. The actual LUMO
state of the adsorbate is the hybridized state, consistingof the highest occupied molecular orbital (HOMO) state,
the LUMO state of the isolated molecule, and the d orbitalsof Pd. Even so, the distortion in the molecule should reflect the shapes of HOMO and LUMO of the isolated molecule unless the molecule is significantly distorted uponadsorption.
The shapes of HOMO and LUMO of a cis-2-butene
molecule are depicted in Fig. 4. Both HOMO and LUMO
have significant parts at the  bonding and antibonding
orbitals, and at the sp3 hydrogen atoms, but not at sp2hydrogen atoms. Indeed, the action spectra revealed thatthe motion of cis-2-butene was actually enhanced forCH3  but was inactive for CH, corresponding to thefact that the molecular orbitals of cis-2-butene have a
distribution at sp3 CH in methyl groups but not at sp2
CH at the C——C bond. Because of the distribution of theelectronic state, CH was not directly excited via inelastic electron tunneling process even though the energy ofthe electron was sufficient for the excitation.
Although the C——C bond is strongly associated with theadsorption states through both HOMO and LUMO, there
were no signs of the excitation of C——C in the action
spectrum. Response to the molecular motion is only available if the vibrational mode can couple with the mode ofreaction coordinate for the motion via anharmonic coupling. Since cis-2-butene is  bonded to the Pd atom,

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PRL 95, 246102 (2005)

PHYSICAL REVIEW LETTERS

week ending
9 DECEMBER 2005‘‘Molecular wire’’ project (03BR1) from the New Energy
Development Organization (NEDO) of Japan.

FIG. 4. Molecular structure (left), and the shapes of HOMO
(center) and LUMO (right) of a cis-2-butene molecule. The
molecular orbitals (MO) were calculated using a semiempiricalquantum-mechanical calculation package MOPAC.

C——C is strongly coupled with the electronic states near
the Fermi level. Thus the vibrational damping throughelectron-phonon pair excitation could be quite efficient[18]. Despite that the energy is sufficient to overcome the—C is invisible inbarrier for the molecular motion, C—
the action spectra of cis-2-butene due to the insufficient
coupling between vibrational modes similar to the case of
CO on Cu(110) [10].
We have discussed the plausible mechanism of vibrational excitation via inelastic electron tunneling in the
STM junction by the use of STM-IETS and action spectroscopy for the vibrationally mediated molecular motionof cis-2-butene. Taking the action spectrum into account,
we have shown here that such an excitation is basicallythrough resonant tunneling [2,17,19,20]. In other words,
the coupling of the vibrational excitation to molecularmotion reflects the feature of the LUMO of the adsorption
state. For cis-2-butene on Pd(110), M-C, CH3 ,C-C, and CH3  are found to be excited, while onlyM-C and CH3  are observed in STM-IETS. In orderto understand the reason why the other modes are notobserved in STM-IETS, a precise theoretical calculation
would be necessary; even though it has been claimed thatthe elastic component contribute to diminish the inelastic
component when the density of states is very high at the
Fermi level [9,19].
The determination of which vibrational mode is responsible for and how they are associated with the various
surface dynamics is essential for achieving mode-selective
control of chemical reactions. A combination of a dynamic
spectroscopic technique such as an action spectroscopy
and a static technique as an STM-IETS would give an
important information to establish the actual dynamics
related to the electron-vibration coupling in a single molecule on the surface.
The authors thank Dr. Hirokazu Fukidome and
Dr. Satoshi Katano for valuable discussion on the assignment of vibrational modes. The present work was supported, in part, by the Grant-in-Aid for Scientific Research on Priority Areas ‘‘Electron transport through alinked molecule in nano-scale’’ (Grant No. 17069006)
from the Ministry of Education, Culture, Sports, Scienceand Technology and International Joint Research Grant*Present address: Institute of Multidisciplinary Research
for Advanced Materials, Tohoku University, 2-1-1
Katahira, Aoba, Sendai 980-8577, Japan.
†
The authors to whom correspondence should be addressed.
‡
Email address: ykim@riken.jp
x
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