Chemical Physics Letters 386 (2004) 196–199

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ZEKE spectroscopy of the copper–pyridine complex

Jun Miyawaki *, Ko-ichi Sugawara

Nanocluster Group, Nanotechnology Research Institute, AIST, Central 5, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan

Received 26 November 2003; in final form 15 January 2004

Published online: 7 February 2004

Abstract

The single-photon zero kinetic energy (ZEKE) photoelectron spectrum of the copper–pyridine 1:1 complex has been observed in

order to investigate the vibrational structure of the corresponding ion. The adiabatic ionization potential of the complex was

measured to be 5.418 eV and the Cu–N stretching frequencies of the neutral and ionized complex were determined to be 195 and

275 cm�1, respectively.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Spectroscopic studies on metal atom/ion–molecule

complexes in the gas phase, which resolve their elec-

tronic, vibrational, and rotational structures, give in-

sight into metal–ligand interactions that play important

roles in many phenomena observed in the field of

physics, chemistry and biology. In our previous work,

we have investigated the vibrational structures of thesilver– and copper–ammonia 1:1 ion complexes by ob-

serving the single-photon zero kinetic energy (ZEKE)

photoelectron spectra and discussed the difference in the

bonding of ammonia to the coinage metals [1,2]. In this

Letter, we report the ZEKE spectrum of the copper–

pyridine 1:1 complex, hereafter denoted Cu–Pyr. Pyri-

dine is the simplest azabenzene, in which one CH group

of benzene is replaced by a nitrogen atom. Therefore, inthe binding between pyridine and a metal atom/ion, the

p donation/back-donation interactions may also play

important roles in addition to the r-donation from the

lone pair of the nitrogen that is the sole interaction in

the CuNH3 complex.

Pyridine molecule adsorbed on coinage metal sur-

faces and clusters has been extensively studied by using

various spectroscopic methods. The Cu–Pyr complexhas been investigated theoretically as a model to un-

* Corresponding author. Fax: +81-298-61-4804.

E-mail address: j.miyawaki@aist.go.jp (J. Miyawaki).

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.01.050

derstand such experimental results [3]. Calculationsshow that the complex has a planar structure with C2v

symmetry where the Cu atom/ion is bound to the ni-

trogen atom of pyridine, indicating that the r interac-

tion with the lone pair electrons on the nitrogen atom is

stronger than that with the p electron cloud [3–5]. The

binding energy of the ion complex Cuþ–Pyr has been

reported from photodissociation [4] and collision in-

duced dissociation [5] experiments. No experimentaldata, however, have been reported so far for the vibra-

tional frequencies of the Cu–Pyr complex. The ZEKE

spectrum of Cu–Pyr observed in the present study pro-

vides for the first time the precise intermolecular

stretching frequencies for both the ionized and neutral

complexes. In addition, the observed adiabatic ioniza-

tion potential (IP) of the Cu–Pyr complex, combined

with the IP of the Cu atom and with the binding energyof the ion complex measured in previous studies [4,5],

provides the binding energy of the neutral complex.

2. Experiment

The experimental setup and scheme are the same as

those used in the previous studies and described in detail[1,2]. The Cu–Pyr complex was synthesized by the re-

action of laser-vaporized metal atom with pyridine va-

por. The metal atom was produced by focusing the

second harmonic output (532 nm) of a Nd:YAG laser

Fig. 1. The single-photon PIE spectrum of the Cu–pyridine complex.

The large increase of the ionization signal located at 43 645 cm�1 is

indicated by an arrow. The IP of the complex is estimated to be

�43 702 cm�1 by taking into account the shift of 57 cm�1 caused by the

electric field (87 V/cm). The stepwise increase in the signal with a

spacing of �270 cm�1 indicates the excitation of the intermolecular

stretching mode in the ionic state.

J. Miyawaki, K. Sugawara / Chemical Physics Letters 386 (2004) 196–199 197

(Continuum Surelite I) on the surface of a copper rod.

Pyridine vapor was mixed with He gas and expanded

through a pulsed nozzle (General Valve 9-279-900) into

a reaction channel (2 mm diameter, 20 mm length). The

Cu–(Pyr)n complexes were produced in the reactionchannel and cooled by a supersonic expansion into the

vacuum. The complexes were introduced through a

skimmer (2 mm) into a detection chamber and then ir-

radiated by a UV laser. The skimmer was located 40 mm

downstream from the exit of the reaction channel,

whereas the laser interception with the complex oc-

curred at 170 mm from the skimmer. The UV laser was a

frequency-doubled dye laser (Lambda Physik SCAN-mate) pumped by a XeCl excimer laser (Lambda Physik

COMPex 100), which was used for both the photoioni-

zation and ZEKE measurements. The linewidth of the

dye laser was �0.15 cm�1 and its wavelength was cali-

brated against the absorption spectrum of 130Te2.

Prior to the ZEKE measurements, in order to locate

the approximate IP of Cu–Pyr, the photoionization ef-

ficiency (PIE) spectrum was observed as the CuþPyrsignal intensity as a function of the laser wavelength. In

the PIE measurements, ionized complexes were ex-

tracted and mass-selected by a Wiley–McLaren type

time-of-flight (TOF) mass spectrometer and detected by

a microchannel plate detector (Hamamatsu F4655).

Then, the experimental conditions, such as the sample

gas concentration, the stagnation pressure, the power of

the ablation laser, and the relative timing of the gaspulse and the ablation and excitation laser pulses, were

optimized to maximize the CuþPyr signal in the TOF

mass spectrum. In the ZEKE experiments, the com-

plexes were first excited to high-lying Rydberg states and

then ionized by a time-delayed voltage pulse applied to

the repeller. Typically, a field of 2.0 V/cm was applied by

a digital delay generator (Stanford, DG535) for 100 ns

with a 3 ls delay from the excitation laser. The ZEKEelectrons produced from the delayed field ionization

were extracted by the same electric pulse in the direction

opposite to the TOF mass spectrometer and were

detected by a channeltron (Murata Ceratron). A small

spoiling DC field (0.2 V/cm) was applied to remove the

prompt electrons from the photoionization, and a

l-metal cylinder was used to encase the ionization

region to shield the ZEKE electrons from the externalmagnetic field. The ZEKE signal capacitatively decou-

pled from the anode was amplified by a preamplifier

(NF BX-31A), averaged by a gated boxcar (Stanford,

SR250), and stored in a personal computer through an

A/D converter.

Fig. 2. The single-photon ZEKE spectrum of the Cu–pyridine com-

plex. Vibrational modes s and r denote intermolecular stretching and

in-plain bending, respectively.

3. Results and discussion

The single-photon PIE spectrum of Cu–Pyr in the

region of 43 200–44 250 cm�1 is shown in Fig. 1. The

spectrum exhibits a clear threshold of the Cuþ–Pyrsignal at 43 645 cm�1 as indicated by the arrow. The IP

of Cu–Pyr is estimated to be �43 702 cm�1, after a

þ57 cm�1 correction for the energy shift induced by the

electric field (87 V/cm) applied in the extraction region.

In addition, the spectrum displays other large steps at�43 915 and �44 185 cm�1. These steps with a �270

cm�1 interval indicate the opening of excited channels of

the intermolecular stretching mode in Cuþ–Pyr as is

seen in other metal-molecule complexes [1].

Fig. 2 shows the single-photon ZEKE spectrum of

Cu–Pyr in the region of 43 330–44 850 cm�1. The

strongest peak at 43 700 cm�1 is assigned to the origin

band, consistent with the PIE spectrum. The spectralline width (FWHM) of this band is �10 cm�1, which

includes unresolved rotational lines and the effect of the

Table 1

Observed band positions and assignment of the ZEKE spectrum of the

Cu–pyridine complex

Position (cm�1)a Assignmentb

43 505 s0143 700 00043 735 r1

1

43 770 s1143 973 s1044 007 s10r

11

44 045 s2144 244 s2044 281 s20r

11

44 315 s3144 515 s3044 545 Cu

44 782 s40aUncertainty �5 cm�1.b Vibrational modes s and r denote the intermolecular stretching

and in-plain bending, respectively.

198 J. Miyawaki, K. Sugawara / Chemical Physics Letters 386 (2004) 196–199

electric field used for ionization. The adiabatic ioniza-

tion potential of Cu–Pyr is determined to be 5.418 eV.

This IP is smaller than that of the free Cu atom by 2.308

eV, indicating significant stabilization upon ionization.

The IP shift is larger for Cu–Pyr than for CuNH3 (1.965eV) measured in our previous study [2] by 0.343 eV,

implying that the binding energy of the ionized complex

is larger for Cuþ–Pyr than for CuþNH3, consistent with

results obtained with collision induced dissociation

(CID) experiments [5].

From the origin band, a progression with an interval

of �270 cm�1 is observed and assigned to the progres-

sion of the intermolecular stretching (denoted ms) mode.By fitting peak positions to T ðmþs Þ ¼ Tþ

e þ xþs ðmþs þ 1=2Þ

�xþs x

þs ðmþs þ 1=2Þ2, the harmonic frequency, xþ

s , and

anharmonicity, xþs x

þs , of this mode are determined to be

275.0� 1.4 and 0.9� 0.26 cm�1, respectively. A small

peak is observed at 195 cm�1 to the red of the origin

band, from which another ms progression is observed.

This band exhibits decreased intensity relative to the

origin band under low-temperature beam conditions andtherefore is assigned to the hot band from the vs ¼ 1

level in the neutral ground state. The intermolecular

stretching frequencies ðxsÞ in the neutral and ionic states

of Cu–Pyr have been calculated by Wu et al. to be 162.0

and 261.1 cm�1, respectively, at the B3LYP/

LanL2DZ,6-311+G(d,p) level. The calculated frequen-

cies are in better agreement with observations for the ion

than for the neutral. The observed Cu–N stretchingfrequency of 195 cm�1 for the neutral Cu–Pyr complex is

smaller than that of the pyridine molecule adsorbed on

the copper colloid surface, 244 cm�1, measured with

surface-enhanced Raman spectroscopy [6]. This fact in-

dicates that the pyridine molecule is more strongly ad-

sorbed on the copper surface than on the single atom.

An additional small peak indicating another msprogression is observed at 35 cm�1 higher in energyabove the origin band. This peak also exhibits di-

minished intensity under low-temperature beam con-

ditions and thus is also a hot band. The Cu–Pyr

complex has the other intermolecular modes, in-plain

bending (mr) and out-of-plain bending (mx) modes, for

which the calculation predicts xr ¼ 113:6, xþr ¼ 167:2,

xx ¼ 36:3, and xþx ¼ 113:3 cm�1 at B3LYP level [3].

These vibrational modes have b2 and b1 symmetries,respectively, thus Dv ¼ 1 transitions are forbidden and

DvP 2 transitions may be weak for these modes.

Therefore, we tentatively assign these bands to the hot

combination bands sn0r11 (n ¼ 0–2), whose frequencies

are expected from calculation to be 53.6 cm�1 larger

than those of the corresponding sn0 bands. The s30r11

band is not clear due to overlap with the sharp peak

at 44 545 cm�1, which arises from the Cuþ ion pro-duced by resonant two-photon ionization [2]. The

observed ZEKE band positions with assignments are

listed in Table 1.

Because the IP shift of Cu–Pyr from Cu is measuredin the present study, the binding energy of the neutral

Cu–Pyr can be derived from that of the Cuþ–Pyr ion by

using the relation of IP(Cu)–IP(Cu–Pyr)¼D0(CuþPyr)

)D0(Cu–Pyr). There are two experimental values

available for the ion complex. The binding energy of

Cuþ–Pyr was first derived from the photodissociation

experiment by Yang et al. [4]. In their experiment, the

parent Cuþ–Pyr complex was photo-excited whilemonitoring the fragment Pyrþ ion resulting from the

dissociative charge transfer state. From the onset of the

Pyrþ signal and difference in the IP of copper and pyr-

idine, the binding energy of Cuþ–Pyr was derived to be

274 kJ/mol. However, owing to the scheme of the ex-

perimental technique, this value may be regarded as an

upper limit.

Rodgers et al. [5] have systematically measuredbinding energies of the Mþ–Pyr complexes for M¼Mg,

Al and transition metals (Sc–Zn) utilizing CID and de-

termined that of Cuþ–Pyr to be 245.9� 10.1 kJ/mol.

Because the obtained D0(Mþ–Pyr) for Co and Ni are in

excellent agreement with those obtained from other ex-

periment, and D0(Cuþ–Pyr) is smaller than the upper

limit obtained from the photodissociation experiment,

their value of D0(Cuþ–Pyr) seems reliable. If we adopt

this value, the binding energy of the neutral species,

D0(Cu–Pyr), is derived to be 23.2� 10.1 kJ/mol. This

binding energy is much smaller than the desorption ac-

tivation energy of a pyridine molecule adsorbed on

Cu(1 1 0) surface, 93.6 kJ/mol [7], consistent with the

larger Cu–N stretching frequency observed for pyridine

adsorbed on copper colloid.

Rodgers et al. compared the observed binding ener-gies of the Mþ–Pyr complexes with those of MþNH3

previously reported. The Mþ–Pyr binding energies for

M¼Ti–Ni were nearly constantly larger than those of

J. Miyawaki, K. Sugawara / Chemical Physics Letters 386 (2004) 196–199 199

the MþNH3 complexes by 26� 9 kJ/mol. In contrast,

the binding energy of Cuþ–Pyr was only slightly (9� 18

kJ/mol) larger than that of CuþNH3. Because Cuþ

cannot accept any p donation from the ligand due to the

fully occupied d orbitals, they attributed the small dif-ference in Cuþ–M (M¼NH3, Pyr) to the larger polar-

izability of the pyridine molecule and p back-donation

in the Cuþ–Pyr complex, and additional �17 kJ/mol

energies for Tiþ–Niþ to the p donation interaction.

Because the r-donation/p back-donation interactions

should occur in the neutral Cu–Pyr complex as well, it is

expected that the binding energy of Cu–Pyr is larger

than that of CuNH3. The binding energy of the neutralCuNH3 is derived to be 47� 15 kJ/mol from

D0(CuþNH3)¼ 237� 15 kJ/mol reported in the previous

CID experiment [8,9] and IP(Cu)) IP(CuNH3)¼ 189.6

kJ/mol measured in our previous work [2]. Although the

central value of neutral CuNH3 is twice as large as that

of Cu–Pyr, it is hard to conclude that D0(Cu–Pyr) is

smaller than D0(CuNH3) considering the large experi-

mental errors. The intensity patterns of the intermolec-ular stretching mode in the ZEKE spectra of Cu–Pyr

and CuNH3 (Fig. 3 in Ref. [2]) are similar; the origin

band and the vs ¼ 1 band have predominant intensities

with the vs P 2 bands being weaker. Because a signifi-

cant difference in the binding energies generally causes a

large difference in the Cu–N bond lengths leading to

different Franck–Condon patterns in the ZEKE spectra,

this similarity may suggest that the binding energies of

the neutral CuNH3 and Cu–Pyr are comparable, rather

than differing by a factor of two. Further experimentsusing alternative techniques are desired to accurately

compare the binding energies of the neutral CuNH3 and

Cu–Pyr complexes.

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