Spectroscopy of clusters at NAIR

Journal of Molecular Structure 480–481 (1999) 45–59

Spectroscopy of clusters at NAIR K.M.T. Yamada* National Institute for Advanced Interdisciplinary Research (NAIR), Tsukuba, 305-8562, Japan Received 25 August 1998; accepted 30 September 1998

Abstract The results of the spectroscopic studies on molecular clusters made in the last five years in NAIR, Tsukuba, Japan, are briefly reviewed. The experimental procedure to obtain the pure rotational and the cluster vibrational spectra of Ar–CO is presented together with the theoretical considerations of the rovibrational energy levels in relation to the potential surface. For the Ar– CH4 complex, the symmetry group and the rovibrational energy levels are discussed in detail as well as the experimental procedure to obtain its IR spectra. In addition, the results from the IR/REMPI double resonance spectroscopy is presented for some clusters containing aniline. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: Ar–CO; Ar–CH4; Aniline cluster; IR spectroscopy; Millimeter-wave spectroscopy

1. Introduction In our institute a research project of cluster science was carried out for five years since 1993. In the present paper I would like to review some results in spectroscopy of clusters from our group. Some of them were carried out in international collaborations with other institutes. First, I would like to comment on the definition of the word cluster which is not so clear in molecular science. The borders of the cluster territory in the directions both to molecules and to condensed bulk materials are exceedingly obscure. Prof. Berry of the University of Chicago once stated, ’’they must be finite aggregates whose composition can be changed by adding or taking away units of the species that comprise them [1]‘‘. He presented a couple of additional conditions to make the definition clearer.

* Tel.: 00 81 2985 42541; fax: 00 81 2985 492549. E-mail address: [email protected] (K.M.T. Yamada)

Anyway it is not possible to define the concept in a few words. Small clusters, which may be investigated in detail by using spectroscopic methods, are close to the paradigm of the word molecule. It may, however, be claimed in many cases that the components of a cluster are bound more weakly each other than to the atoms in the component molecules. In our group, we have limited our effort to study such weakly bound complexes which are not stable in the normal experimental conditions. As a consequence of the weak bonds, internal motions of clusters involve slow and large amplitude modes. For making the discussion more specific, we bring into focus the Ar–CO complex in the next section. It is one of the simplest clusters. The frequencies of the cluster-vibrations of this complex are very low; they are 12.014 cm ⫺1 for the stretching mode and 18.110 cm ⫺1 for the bending mode [2]. The cluster vibrations, not only for the Ar–CO but also for many other clusters, are often observed in the terahertz region or far infrared (FIR) region. The

0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00652-8

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the center of mass of CO and Ar. In the figure we denote the angular momentum for the rotation of CO by j, which represents the internal rotation of the complex. The end-over-end rotation can be expressed by the angular momentum N, where we consider the CO molecule as an atom at its center of mass. Assuming the CO part is rigid, the rovibrational Hamiltonian of the complex can be expressed as p^2 H^ ˆ ⫹ BCO j^2 ⫹ BS N^ 2 ⫹ V…R; u†; 2m

…1†

where

mˆ

Fig. 1. Structure of the Ar–CO complex is illustrated with the definition of several coordinates and angular momenta (see text).

mAr …mC ⫹ mO † ; mAr ⫹ mC ⫹ mO

BCO ˆ h=…8p2 mCO r2 CO†;

…3†

BS ˆ h=…8p2 mR2 †;

…4†

with

mCO ˆ experimental procedure and the nature of rovibrational energy levels are presented in Section 2. For the species with large amplitude vibrations, the use of permutation-inversion group [3] is essential. Its application to the CH4 –Ar complex is discussed together with observed infrared spectra in Section 3. Another specific problem in the spectroscopic studies of clusters is the need of highly sensitive spectroscopic technique, because of the difficulties in producing enough amount of samples. The resonance-enhanced-multiphoton-ionization spectroscopy combined with the time-of-flight mass-analysis (REMPI-TOF), one of the most powerful methods, was successfully applied in our laboratory to a few kinds of molecular clusters. The results of the IR/ REMPI-TOF double resonance experiments are presented in Section 4.

2. Rovibrational energy of Ar–CO complex 2.1. Rovibrational Hamiltonian for Ar–CO The position of Ar relative to CO can be represented by two variables, the radial coordinate R and the angle u as given in Fig. 1; the vector R connects

…2†

mC mO : …mC ⫹ mO †

…5†

The term p^ represents the kinetic energy of the cluster stretching motion. BCO is the rotational constant of CO, and BS is the rotational constant of the complex system in a diatomic approximation. The potential V(R, u ) hinders the internal rotation of CO and regulates the cluster stretching vibration v3. The angular momentum j is in the plane perpendicular to the CO molecular axis, i.e., x 0 –y 0 plane in Fig. 1 with nonvanishing x 0 - and/or y 0 - component. Similarly N is in the x–y plane. The total angular momentum J is J ˆ j ⫹ N:

…6†

If the potential barrier for the internal rotation is very low, the rovibrational energy is expressed as E…v3 ; j; N† ˆ G…v3 † ⫹ BCO j…j ⫹ 1† ⫹ BS N…N ⫹ 1†: …7† Since BCO q BS in the present case, the rotation and internal rotation structure of the complex is just the same as that of the rotational monomer CO associated with the fine structure described with N quantum number in this case. In case the potential barrier for the internal rotation of CO is high, the complex can be handled as an asymmetric-rotor. The two degrees of freedom of j,

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Fig. 2. The radial cross section of the potential surface is presented for the angle u fixed at p /2; the points are MP4 calculation values reported in Ref. [4] and the broken line represents the analytical approximate using the Lennard-Jones 12-6 potential form.

jx 0 and jy 0 ; change their physical meaning to one bending vibration n 2 and one end-over-end rotation along the z-axis, jz. The zeroth order Hamiltonian may be written as H^ ˆ H^ b ⫹ H^ s ⫹ AJ^ z ⫹ BJ^ x ⫹ CJ^y ⫹ D…J^z J^x ⫹ J^ x J^ z †; …8† where j^z ˆ j^z : The last term disappears if we choose principal inertial axes properly. The terms H^ s and H^ b represent the cluster-stretching and cluster-bending vibration, respectively. If we assume the equilibrium structure of the complex to be very close to the MP4 prediction, Fig. 1, then A ⬃ BCO ; B ⬃ C ⬃ BS :

…9† …10†

2.2. Potential energy function for Ar–CO A goal for the spectroscopic study of the Ar–CO

complex is to determine the potential energy function V(R,u ) experimentally. It has, however, not yet been obtained, and there is only a prediction based on ab initio calculations reported by Shin et al. [4]. Their radial potential at u ˆ p=2; i.e., V…R; p=2†; is reproduced in Fig. 2, where the points are their values calculated by MP4. The broken line represents an analytical approximation for those points assuming the Lennard-Jones 12-6 potential form. The potential ˚ in this figure, minimum, ⫺ 98.16 ⫺1, is at R ˆ 3.758 A which is very close to the global potential minima, ˚ and u ˆ 98⬚ at which is located at R ˆ 3.743 A their MP4 level calculation. The binding energy predicted for the complex is not too small, and we can expect the complex being stable for the temperature lower than 140 K. The line connecting the radial potential minima for every angle u define an orbit of Ar around CO with the minimum energy in the classical sense. Needless to say, the orbital motion of Ar around CO is equivalent to the internal rotation of CO of this complex. The

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Fig. 3. The cross section of the potential surface along this potential valley is presented, which regulates the internal rotation of the CO part.

cross section of the potential surface along this potential valley is presented in Fig. 3, where the MP4 [4] calculation is marked by points, and its analytical approximation with V…Rmin ; u† ˆ V0 ⫹ V1 cos…u† ⫹ V2 cos…2u†

…11†

is drawn by the broken line. The potential barriers for the internal rotation are very low. The barrier to the Ar–C–O linear structure at u ˆ 0 is higher than that to the Ar–C–O linear structure at u ˆ p /2. Fig. 3 suggests that the CO part can rotate freely for the levels higher than about 50 cm ⫺1 from the potential bottom. For the levels populated in a cryogenic temperature, e.g. at 10 K, the internal rotation is hindered by the potential barrier, and the CO part undergoes a large amplitude bending vibration. Those low energy states are expected to be very similar to those of a quasilinear molecule with a small potential hump at the C–O–Ar linear configuration. Thus, it may be appropriate to use the asymmetric top notations for this complex, since so far

the observations are limited to those very low energy levels. 2.3. Spectroscopic observations of Ar–CO De Piante et al. reported the infrared (IR) spectrum of Ar–CO for the first time without any explanation [5]. Since McKellar et al. published the first analyzed infrared spectrum [6], the spectroscopic study of this complex was carried out intensively in the infrared region [7–10] and also in the microwave (MW) and millimeter wave (mmW) region [2,11–14]. The infrared transitions observed so far are allowed by the transition dipole of the CO part; the rovibrational band associated to the CO stretching vibration was observed and analyzed. The pure rotational transitions are allowed by the permanent dipole moment of CO. Since the molecular axis of CO in the complex, which is the axis of the permanent dipole moment, is almost perpendicular to the principal a-axis ( ⬃ z-axis), the a-type transitions

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Fig. 4. The NAIR mmW/sub-mmW spectrometer combined with a supersonic jet apparatus is shown schematically.

are very weak, which were observed for the first time by Ogata et al. [11] using the pulsed-jet Fourier transform microwave (FTMW) spectroscopy which is known as a very sensitive technique. In contrast to the a-type transitions, the b-type transitions appearing in the mmW region are expected to be strong because the molecular axis of CO is almost parallel to the molecule-fixed b-axis ( ⬃ x-axis). Ja¨ger and Gerry

Fig. 5. A b-type transition of Ar–CO, 616 ← 505 at 93 GHz, observed by the NAIR mmW/sub-mmW spectrometer combined with a supersonic jet apparatus is shown.

[12] detected two b-type rotational transitions by mmW-FTMW double resonance and determined the A rotational constant. The b-type rotational transitions are so strong that they can be observed by direct absorption spectroscopy. Hepp et al. have measured a number of b-type transitions by a mmW spectrometer combined with a supersonic jet apparatus [2,13]. In addition to those b-type rotational spectra, they have observed the vibration–rotation transitions of the cluster bending vibration in mmW. An apparatus similar to that used by Hepp et al. in Ko¨ln was constructed in our laboratory and is illustrated in Fig. 4. The mmW or sub-mmW radiation is focused at the molecular jet, about 1 cm downstream from the nozzle. In order to increase the sensitivity, double modulation technique is introduced; the frequency of the radiation is modulated with a 10 kHz bipolar squarewave, and the molecular jet is modulated by a pulse nozzle at 140 Hz. The detected signal was demodulated first by a lock-in amplifier at 20 kHz (2f detection for the source frequency modulation, which yields second derivative signal). The time constant for the PSD is set to 1 ms which provides sufficiently wide band width for passing the 140 Hz jet modulated signal. Using a two-channel boxcar

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K.M.T. Yamada / Journal of Molecular Structure 480–481 (1999) 45–59

Fig. 6. The Ka-rotational energy levels for the lowest rovibrational states are illustrated. The J fine structure is not shown here for making the energy level diagram simpler. The levels coupled by inertial asymmetry interactions, by `-type interactions and a possible Coriolis interaction are indicated.

integrator in succession, which is synchronized to the pulse nozzle, the difference signal for the molecular jet on and off is obtained. A b-type transition of Ar– CO observed in NAIR is shown Fig. 5. 2.4. Observed energy levels of Ar–CO The Ka-rotational energy levels for the lowest rovibrational states are shown in Fig. 6. All the energy of the levels were determined by the mmW

measurements [2], except for the excited clusterstretching vibrational state, v3 ˆ 1, the energy of which is derived from the IR observations. A conspicuous difference from normal molecules can be noticed in the energy level diagram; the energies of the two cluster vibrations, cluster-bending n 2 and cluster-stretching n 3, are extremely low as mentioned in Section 1 of this paper. The spectroscopy in terahertz region, which is still under development, is obviously of considerable importance for the cluster science.

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Table 1 Transformation properties of localized wave functions for the representative operators

Fig. 7. Two possible configurations of Ar–CH4 complex are illustrated. The upper one is of C2v symmetry and the lower of C3v. The ab initio calculations prefer the C3v configuration, the lower.

Hepp et al. [2] reported the difficulty in fitting the observed ground state transitions with Watson’s reduced Hamiltonian, especially for the K-type doublings. This anomaly is partially caused by the quasilinear nature of this complex. As shown in Fig. 6, the Ka ˆ 2(e) level of the ground vibrational state is pushed up by the usual inertial asymmetry interaction

Conformer

E^

123

(14)(23)

W X Y Z

W X Y Z

W Z X Y

Z Y X W

(1423)* Y Z X W

(23)* W Y X Z

with the Ka ˆ 0 level. At the same time this level is pushed down by the `-type interaction with the Ka ˆ 0 level of the v2 ˆ 1 state; this interaction is the `-type resonance in a linear molecule. This interaction scheme holds for higher Ka levels and causes significant perturbation to the asymmetric-top rotational Hamiltonian. It should be interesting to mention the degree of quasilinearity of Ar–CO using the quasilinearity parameter g 0 [15],which is defined in the present case as

g0 ˆ 1 ⫺ 4

E…lowest state with K ˆ 0† : …12† E…lowest excited state with K ˆ 0†

The value of the g 0 is 1 for an ideal bent molecule, ⫺1 for the ideal linear molecule. Bunker and Howe [16] modified this parameter to extend the definition for the case of internally rotating triatomic molecules; for a free internal rotor g 0 ˆ 3. From the observed data we can derive g 0(Ar–CO) ˆ ⫹ 0.105, which suggests that the Ar–CO is very quasilinear.

3. Symmetry properties and spectra of Ar–CH4 complex 3.1. Molecular symmetry group of Ar–CH4

Fig. 8. Four equivalent configurations for the C3v Ar–CH4 complex are illustrated (see text).

Ab initio calculations [17] suggest that the argon– methane complex has an energy minimum of the potential energy surface at C3v position with an Ar atom fronting towards a face of tetrahedron formed by the hydrogen atoms of a methane molecule, the lower picture in Fig. 7. Because of the symmetry of CH4 there are four equivalent stable forms. In the present paper, we denote those four equivalent conformers by W, X, Y, and Z as shown in Fig. 8, where W has an Ar atom facing to the plane of the hydrogens 1–2–3, X to the plane of 1–2–4, Y to the plane of

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3.2. Tunnel splittings in Ar–CH4

Fig. 9. Structure of the Ar–CH4 complex is illustrated with the definition of several coordinates and angular momenta.

1–3–4, and Z to the plane of 2–3–4. For understanding the spectroscopic behavior of the complex, it is meaningful to consider two limiting cases; low and high barrier cases. In case the potential barriers between those four potential minima are low enough to allow frequent tunneling, the Ar–CH4 complex belongs to the same molecular symmetry as CH4 itself, i.e. Td(M). The ^ 8…123†; feasible symmetry operators are {E; 3…14†…23†; 6…1423†*; 6…23†*}; where the permutation operators substitute the numbered hydrogen nuclei [3]. Table 1 summarizes the functions of the operators representing the classes. The possible symmetry species are non-degenerate A1 and A2, doubly degenerate E, and triply degenerate F1 and F2. In case the potential barriers are so high that the tunneling from one configuration to another can be ignored, the four conformers, W, X, Y, and Z, are considered to be of different species; each of those is a symmetric top with the three-fold rotation axis along C–Ar. The four species are equivalent (degenerate) and are equally abundant. Symmetry operators which combine the different species are not feasible any more. Thus, the feasible operators are ^ 2…123†; 3…23†*}; and the molecular symmetry is {E; (M) which is a subgroup of Td(M). The possible C3v symmetry species are non-degenerate A1 and A2, and doubly degenerate E. It is useful to know that the triply degenerate F1 of Td(M) is cracked into A2 ⫹ E in C3v and the F2 into A1 ⫹ E.

According to Hougen’s method [18], the energy splitting owing to the internal rotation can be roughly estimated analytically as follows. Assuming that the complex has energy minima at C3v positions as shown by the lower picture in Fig. 7, we define basis functions cW ; cX ; cY ; and cZ to be the wave functions representing the motion of Ar atom localized at the conformer W, X, Y, and Z in Fig. 8, respectively. The energy matrix of the Ar–CH4 cluster can then be expressed as a 4 × 4 matrix: 1 0 cW E T T T C B C cX B BT E T T C C; B …13† C B C cY B T T E T A @ cZ T T T E where the diagonal elements E represent the four levels which are degenerate if the potential barrier is infinitely high. The off-diagonal elements T represent the interaction energies (tunneling energy) between the neighboring potential minima owing to the finite potential barrier, which are equivalent for all, because each conformer is just the next neighbor of others. By transforming the basis functions into the following linear combinations,

c⫹⫹⫹⫹ ˆ

1 …c ⫹ cX ⫹ cY ⫹ cZ †; 2 W

…14†

c⫹⫺⫹⫺ ˆ

1 …c ⫺ cX ⫹ cY ⫺ cZ †; 2 W

…15†

c⫹⫹⫺⫺ ˆ

1 …c ⫹ cX ⫺ cY ⫺ cZ †; 2 W

…16†

c⫹⫺⫺⫹ ˆ

1 …c ⫺ cX ⫺ cY ⫹ cZ †; 2 W

…17†

the energy matrix, Eq. (1), is diagonalized analytically as 1 0 c⫹⫹⫹⫹ E ⫺ 3T 0 0 0 C B c⫹⫺⫹⫺ B E⫺T 0 0 C C B 0 C …18† B C B C c⫹⫹⫺⫺ B 0 0 E ⫺ T 0 A @ c⫹⫺⫺⫹ 0 0 0 E⫺T Since the interaction energy T is expected to be

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3.3. Rovibrational Hamiltonian for Ar–CH4 The overall rotation of the complex is expressed by three Euler’s angle a shown in Fig. 9, b (rotation about the space-fixed Z-axis), and g (rotation about the molecule-fixed z-axis). The attitude of methane relative to the molecule-fixed axis-system (Ar–C axis is the z-axis) is given by two angle u in Fig. 9 and x (rotation about the top-fixed z -axis). The third angle f of the Euler’s angle which represent a rotation of the CH4-fixed (top-fixed) axis-system relative to the molecule-fixed axis-system is equivalent to that given before. The relative motion of CH4 molecule and Ar atom is then fully determined by u , x , and a distance R of Ar and C atoms. Similar to the case of Ar–CO, Eq. (6), the total angular momentum J is the vector sum of the angular momentum N and the angular momentum j of CH4 as shown in Fig. 9. Thus, as presented by Howard and coworkers [19,20], the zeroth order Hamiltonian for rovibrational motions can be written as ^ 2 ⫹ 1 p^2s ⫹ V…R; u; x†; H^ ˆ H^ CH4 ⫹ B…J^ ⫺ j† 2m

Fig. 10. The j ˆ 1 state of the Ar–CH4 complex split into three levels by the coupling of j and N as in the case of 3S diatomic molecule (on the left). In case the potential barrier for the internal rotation is high, the energy level diagram should look like that of a C3v symmetric top (on the right).

negative in most cases [18], the lowest level is a nondegenerate state c⫹⫹⫹⫹ ; and three-fold degenerate state should be the counterpart of the tunnel splitting. The symmetry of the new basis functions can be derived using the transformation properties listed in Table 1; c⫹⫹⫹⫹ is of A1, and the three functions, c⫹⫺⫹⫺ ; c⫹⫹⫺⫺ ; and c⫹⫺⫺⫹ ; span a three dimensional representation of F2. Because the cluster has four equivalent potential minima, each level should split into four, but three of them are degenerate by symmetry. If the complex has energy minima at C2v positions as shown in the upper picture of Fig. 7, it can be shown in a similar manner that the six-fold degeneracy in the ground state will split into A1, E, and F2.

…19†

where HCH4 is the Hamiltonian of the free CH4 molecule, B is the rotational constant of the complex for the x and y axes, m is the reduced mass for the cluster stretching motion, and p^ s is the conjugate momentum of the cluster stretching coordinate R. In the free-internal rotor approximation the potential V is independent of the angles, and the energy levels of CH4 monomer in the ground state embody the energy levels of the internal-rotation of the cluster. For the ground state of the cluster stretching vibration, each of the CH4 monomer levels are accompanied by the rotational fine structure, which is approximately BN(N ⫹ 1) where the quantum number N represents the angular momentum of the end-over-end rotation. Since the vector N is perpendicular to the z-axis, the z-component of the total angular momentum, J^ z ; ^ J^ z ˆ j^z : originates purely from j; The rotation–internal-rotation wave functions, c , can be written as products of the rotational wave functions of methane, cj ; and the diatomic type rotational wave function, cN : Consequently the symmetry of c is given as the product of the that of cj ; which is known for methane, and cN ; which is A1 for an even N and A2 for an odd N.

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Fig. 11. A portion of the IR spectra of Ar–CH4 observed by a diode laser spectrometer in Ko¨ln is reproduced.

3.4. Splitting in j ˆ 1 State in Ar–CH4 Since J ˆ N ⫹ j; N ⫹ j ⫺ i; …; j N ⫺ jj; each nonzero N state split into 2j ⫹ 1 sublevels by the coupling of angular momentum and the anisotropy of the potential. The splittings for the j ˆ 1 state are illustrated in Fig. 10, where analogy to the case b diatomic molecule of 3S is applied on the left for the free internal rotor approximation; j ˆ 1 corresponds to the electronic spin S ˆ 1. In case the potential anisotropy is high (the potential barrier for the internal rotation is high), the complex obtains the nature of a C3v symmetric top, where K is a good quantum number. Then the energy level diagram organized with K, which is a case a analog, is better for understanding; the levels of j ˆ 1 state should be correlated with K ˆ 0 and K ˆ ^ 1 levels in the C3v limiting case on the right side of Fig. 10. The energy level correlation is not so simple and clear as in the case of linear–bent correlation [15], and we have to emphasize more in this case that the wave functions of the levels with same J and same symmetry can be mixed. 3.5. Observed spectra of Ar–CH4 Since this complex has vanishingly small electric dipolemoment, it is very hard to observe its rotational spectrum in the MW or mmW region. The IR spectra of Ar–CH4 associated with the degenerate CH stretching mode, n3 ; were recorded first by McKellar using a low temperature long path cell [21]. Using the jet cooling technique [22,23] the band was investigated in much lower temperatures. However,

presumably owing to the predissociation of the complex the lines are broad and it was not easy to analyze the spectra in detail. Contrarily, the band associated with the degenerate CH bending mode, n4 ; were observed in Ko¨ln clearly as reproduced in Fig. 11 [24–26]. The spectrometer used is essentially the same as given in Fig. 4; the radiation source is replaced by a tunable diode laser system, a slit nozzle, and a White-type multi-reflection optics were applied in this case. The absorption lines shown in Fig. 11 are the transitions associated with the R(0) rovibrational transition of methane; (v4, j) ˆ (1, 1) ← (0, 0). The upper state is of j ˆ 1 and the levels split into three as discussed before. Thus the j ˆ 1 ← 0 subband splits into three components. In the free internal rotor approximation, the radiative excitation of N is very difficult because the transition moment is localized on the methane part. Thus it is expected that strong transitions should be of DN ˆ 0. The three components of DN ˆ 0 transitions of this subband look like Qbranches, and are assigned to the DJ ˆ 0, ^ 1 transitions, i.e., R-, P-, and Q- branch of the subband. The internal rotation of methane in this complex seems to be fairly free, and so far we have not obtained enough information for the potential anisotropy.

4. IR/REMPI double resonance spectroscopy 4.1. Principles of REMPI spectroscopy A high barrier which blocks the spectroscopic

K.M.T. Yamada / Journal of Molecular Structure 480–481 (1999) 45–59

Fig. 12. The apparatus for the IR/REMPI double resonance spectroscopy used in NAIR is illustrated. The vacuum chamber is composed of a supersonic jet expansion chamber for forming clusters and the TOF mass spectrometer. The excitation and ionization UV laser, as well as the IR pumping laser, is focused on the molecular beam in the ionization region of TOF-MS.

studies on clusters is the difficulty to produce enough amount of clusters required for spectroscopic observations. To develop a highly sensitive method is essential for making progress in the cluster science. Resonantly enhanced multiphoton ionization (REMPI) spectroscopy is one of such highly sensitive methods. The principle of REMPI spectroscopy is shown in Fig. 12. A neutral molecule or cluster in the ground electronic state S0 is ionized by irradiating the ultraviolet (UV) laser light through multiphoton process. This ionization process is enhanced when the UV laser frequency coincides with an electronic transition to the excited state S1. Thus, the ion current produced by the photoionization reflects the absorption spectrum of S1 ← S0. The detection of ion current is much easier than detecting weak optical absorption, and thus the REMPI spectroscopy is known as a very powerful tool for detecting samples of very low concentration. By combining time-of-flight mass (TOF-MS) spectrometry in ion detection, we can

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obtain mass-selected spectra, which is an additional advantage of this method. However, electronic spectra show usually complicated band structures and it is not easy to analyze, especially when large amplitude vibrations are involved as in most cases of clusters. In addition it is not easy to increase the spectroscopic resolution in UV region. Use of IR/REMPI double resonance is an elegant method to extort valuable information from the complicated spectra. By pumping an infrared transition form the ground state by an IR laser as shown in Fig. 13, the ion signal is inhibited. Thus, the IR spectra can be obtained by scanning IR laser by observing ion current where the UV laser frequency is fixed at a resonance of S1 ← S0. This technique is utilized for clusters bound weakly by van-der-Waals interactions or hydrogen bonds [27–30]. In our group we have carried out the spectroscopic studies of clusters containing aniline. Our interest is in the hydrogen bonds formed by amino-group, which play an important role in the structures and dynamics of biological materials. 4.2. Experimental procedure for aniline clusters The apparatus we use in NAIR for observing various clusters with aniline is shown schematically in Fig. 12 [31]. The vacuum chamber is composed of two differentially pumped rooms; one is the supersonic jet expansion chamber for forming clusters in cryogenic temperatures; gas mixtures, containing aniline and guest molecules diluted in helium, are injected through a pulse valve operated at 10 Hz with pulse duration of 250 ms. The clusters formed in jet are introduced into the second chamber, the TOF-MS part, through a skimmer. The UV laser light pulse for excitation and ionization, which is generated by frequency doubling of a dye laser using KDP crystal, is focused on the molecular beam in the ionization region of the TOF-MS. The line width of the laser is about 0.1 cm ⫺1 with pulse width of 5 ns. The IR pump laser light is generated by the difference frequency generation using a dye laser and a Nd : YAG laser, and focused on the molecular beam at the spot where the UV light is focused. The IR pulse width is 6 ns. The produced ions are analyzed by a straight TOF-MS. The mass resolution is about 200 at m/z ˆ 100. The ions are

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Fig. 13. The principle of IR/REMPI double resonance spectroscopy is illustrated. A UV laser, in resonance with S1 ← S0 transition, ionize cluster is by a multiphoton (in this case two photons) process. The produced ions are mass selected by a TOF mass spectrometer and detected. The ion signal is inhibited by pumping an infrared transition by an IR laser.

detected by a MCP detector and the ion current is recorded by a digital oscilloscope. The timing of the pulse valve and lasers is controlled by a delay pulse generator. The cluster beam is irradiated by the IR laser 50 ns before the UV laser light arrives at the spot. 4.3. Observed spectra of aniline clusters Fig. 14 compares the IR spectra in the NH stretching region of aniline monomer, dimer [32], aniline–benzene [33], and aniline–pyrrole [34] as observed by IR/REMPI double resonance spectroscopy. The two absorption bands shown in the monomer spectrum are rotationally unresolved absorption bands of symmetric (lower frequency) and anti-symmetric (upper) NH stretching vibration. In the three clusters, the following three phenomena can be noticed: 1. both bands are shifted towards red by fairly large amount, 2. the separations of the two bands are almost equal for the three clusters, and are slightly narrower than that of monomer, and

3. only two bands are observed in aniline dimer where at most four bands may be observed. The first two facts suggest that the NH2-group of aniline is involved in the cluster bonding, and the two hydrogen atoms plays equivalent role. The third fact suggests that the two NH2-groups in the dimer are in equivalent positions. Based on those observations, with a help of quantum-chemical calculations, we concluded that these three clusters are formed by H– p bonding; the two hydrogen atoms of NH2group are bound by the p electrons of the aromatic ring of the guest molecule. Similarly shown in Fig. 15 are the IR spectra in the NH-stretching region observed for Aniline–NH3 [35] and Aniline–TEA [36], where TEA means triethylamine. In contrast to the three clusters discussed before, the shifts of the lines are not so similar for the two NH bands; the lower one shifts larger in red than the upper one for both clusters shown in the figure. This fact suggests strongly that the guest molecule is bound with one of the hydrogen atom of the NH2-group of aniline. The reason for this conclusion is as follows: In aniline monomer, the doubly

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near the local mode frequency which is the center of the two bands in the monomer spectrum. The IR spectra of Aniline–X were investigated in a similar manner in our group, with X ˆ Ar and Ar2 [31], with X ˆ N2, CH4, CHF3, and CO [37], and with X ˆ cyclohexane [33].

5. Discussion

Fig. 14. The IR spectra in the NH stretching region are compared for aniline monomer (top), dimer (second), aniline–benzene (third) and aniline–pyrrole (bottom) as observed by IR/REMPI double resonance spectroscopy. The two absorptions in each spectrum are assigned to the symmetric and anti-symmetric NH stretching vibrations of aniline. The strong line near 3500 cm ⫺1 in the bottom spectra is assigned to the NH stretching vibration of pyrrole.

degenerate NH-stretching local modes split into two normal modes, symmetric and anti-symmetric, by a harmonic interaction of the first order. If a guest molecule is attached to one of the hydrogen atom, the vibrational energy of the bonded NH should be shifted largely, most probably to the low energy, and thus the degeneracy of the local modes is removed, changing the first order interaction between the two into the second order. Thus the NH stretching vibration for the free NH remains almost unperturbed, and appears

The spectroscopy of clusters is an attractive field for high resolution spectroscopy. Numerous papers concerning to the spectroscopic study of molecular clusters were published especially in this decade, and many papers are surely in the process of publication just now. It is, therefore, not easy review on this subject for the present, and in this paper I limited myself to report only the results from group. As presented in Section 2 and Section 3, the spectra of clusters often requires re-examination of the vibration–rotation treatments. Because the large amplitude cluster vibrations violates the approximations based on the small amplitude vibration, the established theory using the harmonic-oscillator-rigid-rotor basis functions cannot well explain the observed spectra. A method to determine experimentally the potential energy surface which governs the cluster vibration, which is a goal of the cluster spectroscopy, has not yet been acquired. The situation of a triatomic case, like Ar–CO in Section 2, is believed to be very close to the goal. Contrarily, simple complexes like Ar– CH4 in Section 3, still require our efforts to fully understand their spectroscopic responses. As reported by Brookes and McKellar recently [38] it is very difficult to understand the spectra of very simple clusters, like CO dimer. I believe, however, that the high resolution spectroscopy is the method to understand the peculiar nature in the spectra of clusters. The development of the experimental method for observing spectra of clusters is ever-progressing. For example, in our group the IR/REMPI double resonance spectroscopy was applied not only for detecting ground state neutral clusters as presented in this paper, but also for detecting cluster ions [35,39,40]; using the same set-up of the apparatus, Fig. 12, the IR pump laser is ignited 50 ns after the ionization UV laser for this purpose. The improvement in resolution for the

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Fig. 15. The IR spectra in the NH stretching region are compared for aniline monomer (top), aniline–NH3 (middle) and aniline–TEA (bottom) as observed by IR/REMPI double resonance spectroscopy. The lower of the two absorption peaks is assigned to the bound NH and upper to the free NH for both aniline–NH3 and aniline–TEA.

REMPI spectroscopy is eagerly required now, and will be achieved in near future, which will be a very promising method to supply spectroscopic data of clusters. Combining many kinds of experimental techniques, including the spectroscopy in the terahertz region, it is expected that a great number of spectroscopic data for a variety of clusters will be supplied from experimental side, which requires a further development in the theory of spectroscopy and the quantum–chemical calculations as well.

Acknowledgements The author would like to express his thanks to

all the spectroscopists of the Cluster Science Group of NAIR, K. Sugawara, T. Nakanaga, F. Ito, J. Miyawaki, I. Morino, K. Kawamata, and many guest scientists, who have contributed to the results presented here. He is also grateful to Prof. G. Winnewisser and his group, especially Dr. M. Hepp, Dr. I. Pak, and Dr. F. Lewen for their contributions to the works presented in Section 2 and Section 3. Some of the results of REMPI spectroscopy presented in the Section 4 are obtained by international collaborations with Prof. B. Brutschy’s group in Frankfurt and Prof. H. Jones’ group in Ulm. The author expresses his sincere thanks to Dr. H. Takeo, who offered him a position in NAIR to study cluster science in the period from 1994 to 1998.

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