Decay energetics of molecular clusters studied by multiphoton mass spectrometry and pulsed field threshold ionization

International Journal of Mass Spectrometry and Ion Processes 13 1 (1994) 21 l-232 0168-l 176/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved

211

Decay energetics of molecular clusters studied by multiphoton mass spectrometry and pulsed field threshold ionization H.J. Neusser”, H. Krause Institut fiir Physikalische und Theoretische Chemie, Technische Universitiit Mtinchen, Lichtenbergstrasse 4, 85748 Garching, Germany

(Received 27 May 1993; accepted 22 July 1993) Abstract New developments in time-of-flight mass spectrometry combined with multi-photon ionization provide hitherto unachievable information on the decay of weakly bound cluster ions. Dissociation thresholds of cluster ions are important for the understanding of their binding and structure. We present two experimental methods that allow the determination of dissociation energies of weakly bound ionic and neutral clusters. Besides the breakdown technique observing the metastable ion efficiency curves, a novel method is presented and described which is based on delayed pulsed field ionization of long-lived Rydberg states close to the ionization energy. In this way state-selected benzene-noble gas ions are produced and their decay is observed in a reflectron mass spectrometer. Results for dissociation thresholds of various molecular dimers and larger complexes of aromatic molecules are presented. The pronounced differences in dissociation energy between the neutrals and the ions are interpreted in terms of charge-transfer resonance interaction in the ionic complex leading to structural changes after the ionization process. Key words: Clusters;

Multiphoton

ionization;

Pulsed field ionization;

1. Introduction

Most of the information on the dissociation kinetics and energetics of molecular ions has emerged from mass spectrometric investigations. The basic principle is the observation of the fragmentation pattern of molecular ions as a function of the energy of the ionizing electrons or photons. In this way, appearance energies for different decay channels of a variety of ions have been determined from breakdown graphs of the fragment intensity [l]. With appropriate kinetic models, dissociation energies can be deduced from the appearance energy. Here statistical models like quasiequili* Corresponding

author.

SSDZOl68-1176(93)03884-O

Reflectron;

Charge

transfer

resonance

brium and RRKM have been successfully applied [2-41. The metastable decay of ions, i.e. their dissociation during their flight through the mass spectrometer, plays an important role as it leads to a clear identification of the parents and the daughters of a particular decay process. Furthermore, the decay time of the ions can be determined from the shape of the metastable mass peaks when the typical flight times in the mass spectrometer are known [5]. For the investigation of decay dynamics it is necessary to observe the decay of ions whose internal energy is well defined. Clear tests of theoretical models are more feasible with energy-selected ions, since, in this way, experiment and theory can be compared with a minimum of averaging. Produc-

212

H.J. Neusser and H. Krause/M.

tion of energy-selected ions turns out to be a nontrivial problem: even if photons of well-defined energy are used for ionization the ions are generally produced with a broad distribution of internal energies spanning the range from zero to the maximum energy above the adiabatic ionization energy. The emitted electrons take away a variable amount of energy and leave behind ions with a broad energy distribution. The shape of the internal energy distribution of the ion is not known apriori since it is a complicated function of Franck-Condon factors and autoionization resonances. A clear situation exists exclusively at the first ionization threshold. Here, no excess energy is left for the emitted electron and the threshold ions are inherently energy-selected. At higher vibrational energy thresholds, in addition to the energy-selected threshold ions, a great number of non-energy-selected ions due to the lower ionization potential are produced. This gives rise to a step-like behavior of the total ion current, each step indicating a new ionization threshold. For experiments with energy-selected ions, the separation of threshold ions from the strong background of non-energy-selected ions, is inevitable. One of the techniques that has been successfully applied is the photoion photoelectron coincidence technique [6,7]. Here, ions that are in coincidence with electrons of low or zero kinetic energy are monitored exclusively. Naturally, the energy resolution of this technique is limited by the monochromator resolution and the electron energy resolution. The hitherto sharpest energy resolution obtained with the technique was some 100meV (x 800 cn-‘). A better resolution is expected if high repetition rate lasers in the UV range become available for ionization. In this report we describe a new technique to produce state-selected ions that is based on the pulsed field ionization of long-lived Rydberg states. Pulsed field ionization allows the observation of highly resolved ion spectra by detecting photoelectrons, as has been demonstrated by Reiser et al. [8], or by monitoring threshold ions

J. Mass Spectrom. Ion Processes 131 (1994) 211-232

[93. In this work we discuss its virtue for the production of state-selected ions, particularly for cluster ions [lo]. It will be demonstrated that the production of ions in various vibrational states is possible and that the decay of cluster ions with low dissociation energy can be observed. The selectivity of the ion production is supported by resonance enhanced two-photon ionization. Resonance-enhanced two- and multiphoton ionization was shown to have particular advantages for mass spectrometry [l 1- 141. Efficient ionization is achieved by stepwise two- or three-photon absorption via a resonant real intermediate state. This resonance enhancement of the multiphoton ionization process leads not only to a drastic increase of the ionization yield but also results in a high selectivity of the ion production. In contrast to a conventional one-photon ionization into the ionization continuum, in resonance-enhanced multiphoton ionization the ionization is controlled by the resonant intermediate state spectrum which often displays sharp structure, even in large polyatomic molecules and small clusters of these molecules. In addition, it is species selective, this being of particular importance for the study of clusters. The goal of this work is to demonstrate that mass spectrometric techniques provide new information on the energetics and kinetics of weakly bound clusters with dissociation energies of less than 1 eV. In section 2.2 we give a description of our experimental setup. Then, results on the dissociation energies of dimers of aromatic molecules and of larger homogeneous clusters are presented. These have been deduced from the measured breakdown of the metastable decay process as a function of the two-photon energy and the ionization energies. Several conclusions on the type of interaction and the resulting structures of the complexes are discussed. In section 3 the pulsed field ionization technique used in this work is described and its application for production of state-selected cluster ions presented. The results yield upper limits for the binding energy of dimers of benzene with noble gases.

H.J. Neusser and H. KrauselInt. J. Mass Spectrom. Ion Processes 131 (1994) 211-232

2. Breakdown measurements 2.1. Introductory

remarks

The measurement of breakdown graphs is a wellestablished technique in the mass spectrometry of molecular compounds. This technique has produced a variety of data on the energetics of molecular systems (see e.g. ref. 15). In this work the selectivity of the resonance-enhanced twophoton ionization is combined with the observation of the metastable decay of small homo and hetero clusters of polyatomic molecules. It will be shown that a wealth of information on the dissociation energy of neutral and ionic dimers and even larger complexes is obtained with this technique. In a dissociation reaction without reversal activation energy, simple relations between the dissociation energy Do of the neutral dimer X2, the dissociation energy E0 of the ionic dimer Xt, and the ionization energies of the monomer X and the dimer X2 exist: &(X,)

= AE - IE(X)

&,(X,f) = AE - IE(X2)

(1) (2)

Here IE(X) and IE(X2) are the ionization energies of the monomer and the dimer, respectively: AE is the appearance energy for the dissociation of the dimer ion Xl into X+ and X. Using these relations the dissociation energies of the neutral and the ionic dimer can be determined from the measured ionization energies of the monomer and dimer and the appearance energy. Eqs. (1) and (2) can be easily extended to hetero dimers and larger complexes. The main problem with the determination of the dissociation energy is a kinetic shift of the appearance energy from this threshold. Generally, this kinetic shift can be expected to be smaller when a slow decay of the ion is monitored. Particularly, for benzene dimers with a relatively small dissociation energy below 1 eV, we have shown that the restricted phase space of the van der Waals modes [16] leads to negligible kinetic shifts [ 171. A general problem in a cluster experiment with

213

supersonic cold molecular beams is the variety of complexes of different sizes and the monomers that are produced simultaneously. It is difficult to distinguish between monomer ions that have been produced by ionization of a neutral monomer and monomer ions originating from the rapid dissociation of the dimer ion. The situation is clear when a metastable decay channel is observed, since monomer ions produced in this way can be well separated from the monomer ions produced by direct ionization of the neutral monomers, e.g. in a reflectron mass spectrometer operated in the partial correction mode [ 18,191 (see below). Moreover, the selectivity of resonance enhanced two-photon ionization provides a means of selectively ionizing, e.g. the dimer, and of suppressing the signal from a direct ionization of the monomer. A necessary precondition for this is that the intermediate states of both species are sharp and well separated from each other, this being the case for a variety of molecules. Another crucial point for the application of the relation in Eqs. (1) and (2) is the ionization energy. Strictly speaking, the adiabatic ionization energy has to be known to find exact dissociation energies rather than lower limits. This is no problem for the monomers since here the ionization thresholds are sharp and can be found with an accuracy of a few reciprocal centimeters. However, it turns out to be a problem for dimers, particularly if geometrical changes from the neutral to the ionic dimer occur (e.g. from a T-shaped to a sandwich structure [20]). Here vertical transitions to higher vibrational states in the intermolecular potential are the strongest ones and the adiabatic transition is weak and its position cannot be identified. As a general rule we may assume that a very sensitive measurement of the ion current leads to ionization energies close to the adiabatic ionization energy. In order to confirm this we recorded photoionization efficiency curves after two-color twophoton ionization using different intermediate states. Principally, one expects that the FranckCondon factors for transitions to the electronic ground state of the cluster ion should change for

H.J. Neusser and H. KrauselInt. J. Mass Spectrom. Ion Processes 131 (1994) 211-232

214

different intermolecular modes of the cluster in the intermediate state. Additionally, it should be mentioned that autoionization resonances, as observed for several van der Waals dimers, [21-231 could strongly enhance the transition probability to the ionic ground state. However, in some clusters with small geometrical changes during the ionization process, sharp onsets of the ionization are observed [24,25]. Here, the technique of pulsed field ionization which will be described below can be applied to find very accurate values of the adiabatic ionization energy.

(He) at a pressure of 1-5 bar. The gas mixture is expanded through a pulsed 200pm diameter nozzle orifice and the central part of the beam is selected by a skimmer before it enters the mass spectrometer in a collinear configuration. The clusters are ionized by resonance enhanced two-color two-photon ionization in the acceleration region of a linear reflectron time-of-flight (RETOF) mass spectrometer [17]. In the linear RETOF the metastable ion intensity for the different decay channels of a hetero cluster ion can be measured with high precision for different decay channels and clusters, since the ion trajectories are independent of the kinetic energy of the observed ions. In order to determine the ionization energy of the clusters, photoionization efficiency curves were recorded by observation of the ion signal of the respective cluster for a fixed photon energy of the first laser, (tuned to the resonant intermediate state) but a varying photon energy of the second laser. To find the appearance energy of a dissociation channel we recorded the intensity of daughter ions that are produced by slow, metastable disso-

2.2. Experimental The details of the experimental setup were described in our previous work [17]. A scheme of the setup is shown in Fig. 1. Benzene homo clusters or heterogeneous van der Waals clusters of benzene (B), para-difluorobenzene (F), toluene (T) and cyclohexane (C) are produced in a supersonic jet expansion of a gas mixture, consisting of 20mbar of each monomer component seeded in noble gas SIGNAL II

ION

DETECTOR

“rep DRIFT

GAS

I

--_-__ __---___ION

“i”

PUMP

1

PUMP

2

lllllll

REGION -___ __--

_-

-_

I, ’iREFLECTOR _-~-=----_ - - I _’ _

_,_

I

BEAM

L_

I

_-_-

I

“r’ -ud uref

PU,MP

3

Fig. 1. Schematic drawing of the linear reflectron mass spectrometer that has been used for the experiments described in this work. The molecular beam produced by a pulsed nozzle is skimmed and collinear to the ion flight paths. The ions are produced by multiphoton ionization by either one laser beam or two laser beams of different color. After acceleration they pass a small hole in the channel plates and are reflected by 180” in the reflecting field towards the channel plates.

H.J. Neusser and H. KrauselInt.

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Ion Processes

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131 (1994) 211-232

ciation of the parent cluster ion in a field-free drift region of the RETOF instrument. The onset of the metastable signal yields the appearance energy of the observed dissociation channel. For the investigation of the appearance energy of a selected decay channel of a cluster ion the linear RETOF is operated in the partial correcting mode. In this mode, daughter ions produced by a cluster decay in the field-free drift region of the RETOF can be observed as separated peaks (drift peaks). This is shown in Fig. 2 for the dissociation of the Bl dimer ion into a B+ ion and a neutral B molecule. In this experiment a delay of 20ns between the first and the second laser pulse was chosen. Therefore, the ion signal originating from twocolor two-photon ionization is well separated

from the signal produced by one-color twophoton ionization through the first laser alone. Both drift peaks due to one- and two-color ionization are marked by arrows in the upper mass spectrum showing the drift peaks on an enlarged scale. Similar mass spectra were recorded for all investigated homo and hetero dimers. The expansion conditions were chosen such that the intensity of larger clusters, e.g. the trimer ion, is 1000 times smaller compared to the dimer intensity. Thus, dissociation from larger clusters into the dimer ion can be neglected. The laser power is chosen so that photoionization of the dimer is maximized and at the same time the ion yield for smaller masses resulting from fragmentation after three-photon absorption is minimized.

B+

B=: C,H,

B+--B;

(v,+vz) B,’

50

60

55 TIME-

OF - FLIGHT

70

65

75

[ PSI

Fig. 2. Part of the time-of-flight mass spectrum of the benzene dimer (Bz) when operating the reflectron in the partial correction mode. The small peaks at 58.5 p (see inset) are due to a metastable decay of the benzene dimer cation (B:) in the drift region of the reflectron mass spectrometer depicted in Fig. 1. Ionization was achieved in a two-laser, two-color experiment with the second laser pulse delayed by 100 ns. The first mass peak originates from an excitation of two photons with hvt , while the second stronger peak is a two color signal due to the absorption of one photon from each laser beam (hut + hvz).

H.J. Neusser

and

H. KrauselInt. J. Mass Spectrom. Ion Processes 131 (1994) 211-232 I-

6’ w’

s,

8.45

8.9

90

90

TWO-PHOTON

9.1

93

ENERGY

9.L

IE

I,

I

I.

8.5

8.55

8.6

Two-Photon

I,

I

8.65

8.7

Energy

,,.I.

8.75

8.8

8 5

[eV]

[eVl

Fig. 3. Breakdown of the intensity of the metastable ion peaks for three dimers, benzene-toluene (BT), toluene-toluene (Tz) and benzene-benzene (Br). The relative intensities are plotted as a function of the two-photon excitation energy. The appearance energy of the metastable decay is taken as the intercept of the breakdown graph with the constant noise level baseline produced by three-photon absorption.

2.3. Dimers of aromatic molecules

The technique described above has been applied to a variety of homo and hetero dimers. A typical result for the breakdown of the metastable intensity is shown in Fig. 3 for the benzene (B2) and toluene (T2) homo dimer and the benzene-toluene hetero dimer (BT) [17]. The appearance energy is determined with an accuracy of about 10meV. Figure 4 displays the onset of the ion current of B2 near the threshold. The arrows indicate the smallest photon energy leading to an ion current above the noise level. The lower trace shows the ion current after excitation of the 6’, S, vibronic state with the first photon. In order to vary the FranckCondon factors for the transition from the intermediate to the ionic ground state in the upper spectrum, an intermediate state including the excitation of an intermolecular vibration [26] was used. The identity of this state is not clear. For this

Fig. 4. Ionization efficiency curves as a function of the twophoton energy hv, + hvz for two different intermediate states of the resonance enhanced two-photon ionization process: bottom, the vs intramolecular mode is excited in the S, intermediate state (hv, = 38564cn-‘); top, in addition to the vs mode, an unassigned intermolecular (van der Waals mode) is excited (hvl = 38586cm-‘). In both cases the same ionization energy is obtained. For interpretation see text.

reason we utilized other van der Waals intermediate states as well, but obtained basically the same result as shown in Fig. 4. Here, both intermediate states yield the same ionization threshold within the error limits of the experiment. Because of this result and the high sensitivity of our experiment we believe that this value is close to the adiabatic IE. All results for the ionization energies and appearance energies are listed in Table 1 together with the deduced dissociation energies DOand E,-,of the neutral and ionic dimers, respectively. We would like to mention that, for the determination of the neutral dimer dissociation energy D,,, the very accurate ionization energies of the monomer are used (see Eq. (1)). Thus, an error due to an uncertainty of the ionization energy can be excluded in this case. It is interesting to discuss the results in a systematic way in terms of the different properties, e.g. polarizability, dipole moment etc., of the

Table 1 Dissociation energies of dimers. Measured ionization energies (IEs), appearance energies (AEs) of the metastable dissociation channels discussed in the text, and dissociation energies Ds and Es of the respective neutral and charged clusters (for definition of Ds and Es see text); the errors represent the reproducibility of the measured values Neutral Dimera

IE (eV)

Main intermolecular I interactions

Dimer cation

AE (eV)

,%I(meV)

Main intermolecular interactionsb

Benxenebenzene Benzenecyclohexane Toluenetoluene Benzenetoluene Pura-DFBPara-DFB Benxeneparu-DFB

8.65 f

0.01

70f 10

Dispersion

9.31 f 0.01

66Of20

CTR

0.02

80f20

Dispersion

9.32 f 0.02

2OOf40

Electrostatic

8.34f

0.01

150 f 10

8.97 f 0.01

630 f 20

CTR

8.42 f

0.01

130f 10

(Benzenebenzene)+ (Benxenecyclohexane)+ (Toluenetoluene)+ (Benzenetoluene)+ (para-DFBpuru-DFB)+ (BenzenePuru-DFB)+

9.12f

8.95 f 0.01

530 f 20

CTR

9.25 f 0.02

380 f 40

CTR/electrostatic

9.24 f 0.02

490 f 40

CTR

DO (mev)

8.87 f 20 8.75 f

0.02

90f20 80f20

a Puru-DFB: Puru-Difluorobenxene. b CTR: charge transfer resonance interaction.

Dispersion dipole-dipole Dispersion dipole-ind. dipole Dispersion Dispersion

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and H. Krause/M.

molecules. Since benzene (o = 10.6A3 [27] and para-difluorobenzene (o = 10.3 A3 [27]) have no dipole moment and nearly the same polarizability, the dissociation energies D,-,of BZ, F2 and BF are expected to be nearly the same. This is in full agreement with our experimental data. Toluene has a small dipole moment (p = 0.38 D [28]) and a slightly larger polarizability (Q = 12.6A3 [27]) compared with those of benzene and p-difluorobenzene. This agrees with the experimental finding that D,, is larger for the BT and T2 dimers. For ionized van der Waals clusters, electrostatic and charge transfer resonance interactions are strongly enhanced, leading to an increased dissociation energy compared to that of the neutral dimers. For instance, for the benzene dimer, the binding energy increases by nearly one order of magnitude from 70meV to 60 meV when the dimer is ionized. Thus, ionization goes along with a strong structural rearrangement of the complex. While the neutral benzene dimer is supposed to have a T-shaped structure, the dimer cation, stabilized by charge transfer resonance interaction, should have a sandwich structure with a parallel arrangement of the two benzene planes. Charge transfer resonance interaction is known to stabilize charged dimers of aromatic molecules. For heteroclusters the strength of this interaction decreases as the difference in the ionization energy of the monomers increases. A large difference in the ionization energy leads to a localization of the charge at the component with the lower ionization energy, and thus to a small charge transfer resonance interaction. Therefore, bonding of the homodimer ions is expected to be stronger than that of the heterodimer ions. From Table 1 it can be seen that indeed the dissociation energies Eo(Bi) and E,,(Tr) are nearly the same and significantly higher than Es(BT+). In the case of Fi the lowering of the dissociation energy is attributed to a partial charge localization at the electronegative fluorine atom, which most likely leads to a decreased charge transfer resonance interaction. It is interesting to study the contribution of the charge transfer resonance interaction to the bond-

J. Mass Spectrom. Ion Processes I31 (1994) 211-232

ing in van der Waals dimer ions in more detail. For this reason we investigated the extreme situation of an heterodimer consisting of an (aromatic) benzene and the nonaromatic cyclohexane molecule. After two-photon ionization benzene-cyclohexane dimer ions (BC+) show a slow metastable dissociation in the drift region of the RETOF leading to benzene daughter ions (B+) and neutral cyclohexane molecules (C). The first laser frequency was fixed to the 6; transition in benzene and the energy of the second photon was varied. The appearance energy of the above mentioned decay was found to be 9.32 eV and the ionization energy 9.12 eV. Using these values, the binding energy of the neutral dimer and the dimer ion are calculated from Eqs. (1) and (2), respectively, to &(BC) = 80meV and Eo(BC) = 200meV (see Table 1). The binding energy in the neutral heterodimer &(BC) = 80 meV is equal or nearly the same as in Bz, BF and F2 dimers. Since all of these molecules (B, F and C) have no dipole moment and their polarizabilities do not differ very much (cyclohexane: cr = 10.9 A3 [27]), this result is in line with simple theoretical arguments. The binding energy in the heterodimer ion (BC+) is Ee(BC+) = 200 meV. If we suppose that no charge transfer resonance interaction contributes to the bonding in BC+ (because of a missing delocalized r-electron system in cyclohexane) the bonding in BC? is purely electrostatic. By comparing the data of the benzene dimer Bz with charge transfer resonance interaction (Eo = 660meV) and BC? without charge transfer resonance interaction (Eo = 200meV), we conclude that the contribution of charge transfer resonance to the binding energies is as much as 400meV in sandwich-like dimers of aromatic components. 2.4. Larger benzene complexes (2 < n < 5) Measurements of the breakdown of the metastable decay efficiency have also been performed for larger benzene clusters up to the pentamer [29]. For these clusters the intermediate state spectra are sufficiently separated so that a selective

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219

J. Mass Spectrom. Ion Processes 131 (1994) 211-232

arrangement of the two benzene molecules and calculated a binding energy of 114 meV employing potential-energy minimization with empirical atom-atom potential functions. Similar results were obtained by de Meijere and Huisken [31]. This structure, however, is at variance with the Tstructure in the benzene crystal. In ab initio calculations a T-structure was found as the most stable structure of the benzene dimer. In an early work of &sky et al. [32] a binding energy of 65 meV was calculated and in a more recent study by Hobza et al. a higher binding energy of 117 meV was obtained [33]. When comparing the theoretical results with the experimental values presented here, it must be recognized that the calculated binding energies yield the energy of the potential minimum D,, whereas the experimental ones represent the lowest vibrational energy level Do (see e.g. Fig. 4 in our previous work [ 171). The results differ by the zero-point energy of the multidimensional van der Waals potential. For the larger clusters, ab initio calculations are not available. Binding energies have been calculated from empirical atom-atom potentials employing a potential energy minimization program [30,31] assuming a triangular structure of the

excitation and ionization of the cluster species under consideration is possible. All results for the ionization and appearance energies and the deduced neutral and ionic dissociation energies are listed in Table 2. In addition, the total binding energies are listed. They are obtained from the sum of the dissociation energies for stepwise evaporation of a monomer. A striking result is the large dissociation energy of the neutral trimer of 200meV as compared to that of the dimer. This suggests a particularly stable structure of the trimer. Assuming that only pair interactions contribute to the binding of the trimer, the maximum binding energy of the trimer should be three times the dimer binding energy, and the measured value Of Dbind(3) = 270 meV appears to be too high compared to the value for the dimer. It is hardly possible to decide whether an additional threebody interaction contributes to the binding in the trimer or whether the deviation is caused by the experimental error. A fourth benzene molecule is bound to the trimer with a somewhat smaller energy of 100 meV. Several theoretical methods have been applied to calculate the binding energy of the benzene dimer. Van de Waal [30] assumed a nearly parallel

Table 2 Dissociation energies of small neutral and ionic benzene clusters. Measured values for the ionization energies (IEs) and the appearance energies (AEs) of the monomer evaporation of benzene clusters (CsHh),(n = 1 - 5). From these values the dissociation energies of the neutral clusters (DO) and the cluster ions (Es) are deduced; the errors represent the reproducibility of the measured values and do not include a systematic error in the values of IE and AE; calculated values D, from the literature, which represent the potential minimum, are also listed Parameter

AE (eV) IE (eV)

(Benzene), 1

2

9.243

9.31 8.65 70 660 70 660

DO WV

_

EO @W Dbind cmev)

_ _

Ebind crnev)

-

0, (meV)

_

For comparison a Reference 33. b Reference 30. ’ Reference 3 I.

of Do with D, see text.

117= I I 4s.’

3 i 0.01 f 0.01 iI0 f 20

8.85 8.58 200 270 270 930 219b 192’

4 f 0.02 f 0.02 f 30 f40

5

8.68 8.55 100 130 370 1060 243b 147=

f 0.02 f 0.02 f 40 f40

G8.61 8.50 f 0.02 ( 60
220

H.J. Neusser and H. Krause/M.

trimer. Results for the trimer are in surprisingly good agreement with our experimental value. This is a strong corroboration of the triangular structure assumed in both theoretical approaches [30,3 1J. New spectroscopic measurements of isotopically substituted benzene trimers also show evidence for a triangular structure [34]. The agreement is less conclusive for the tetramer. Here, the value of van de Waal (243meV) [30] for a tetrahedral structure is more than twice the experimental value. The experimental value is close to the dissociation energy calculated by de Meijere and Huisken [3 l] for a different structure. Here it is assumed that the fourth benzene molecule is added to the strongly bound trimer in such a way that the angle between the plane of the attached monomer and one neighboring molecule of the trimer ring is close to the dimer angle. Our experimental results seem to favor this structure for energetic reasons. For the pentamer, only one theoretical value exists which has been found for the ditetrahedral structure. The much smaller experimental value does not support this structure. Contrary to the neutral binding energies, the ionized cluster dissociation energies show a monotonic decrease with increasing cluster size. As already mentioned above, the binding energy of the dimer cation is nearly one order of magnitude larger than that of the neutral dimer. This was attributed to a strong charge transfer resonance interaction in the dimer ion, typical for a sandwich structure of the benzene dimer ion [17,35-381. The binding energy of the third benzene molecule in the trimer ion is less than half the binding energy of the dimer cation. Badger and Brocklehurst [36] predicted that charge transfer interaction should be responsible for the binding in the trimer ion, if one assumes a triple sandwich structure. A simple Htickel molecular orbital calculation, assuming a delocalized charge in the trimer and the tetramer ion, yields relative binding energies that are in excellent agreement with the experimentally observed increase of the total binding energy [29]. From this we conclude that charge transfer resonance interaction is the dominating contribution to

J. Mass Spectrom. Ion Processes 131 (1994) 211-232

the binding energy in benzene cluster ions up to the tetramer. The main assumption in the Htickel calculation is a delocalization of the charge that points to a non-cyclic sandwich structure of the cluster ions. It is interesting to compare the sandwich structure of the trimer ion with the proposed cyclic structure of the neutral trimer. Whereas for the charged trimer the strongest stabilization is achieved by a charge transfer resonance (Ebind(3) = 930meV) in a non-cyclic structure, for the neutral trimer the largest dispersive binding energy (&ind (3) = 270 meV) is achieved in a cyclic configuration. Experimental values for the binding energy have been obtained from high-pressure mass spectrometry (HPMS) experiments [35,39]. The most recent value of Meot-Ner et al. is &,(2) = 740 f 65 meV [35]. This is in reasonable agreement with our result. New results of Hiraoka et al. [39] point to a somewhat higher binding energy for the dimer of E,,(2) = 894 f 43 meV and a binding energy of the trimer ion of E0(3) = 338 f 22 meV. The latter is in reasonable agreement with our result of Es(3) = 270 f 40meV. 3. Production and decay of state-selected cluster ions 3.1. Introductory remarks

A common characteristic of the clusters studied in the preceding section is that they undergo strong structural changes after the ionization process. For example, in the benzene dimer the T-shape structure of the neutral complex changes to a sandwich structure in the ion. As a consequence, the vertical ionization process leads to the excitation of many intermolecular modes in the ion and the adiabatic ionization transition is weak. Some of the six van der Waals modes have low frequencies and produce a smoothly increasing ionization efficiency curve with densely packed ion states rather than a steplike behavior of the ion current with each new channel separated from the preceding one. For this reason there is little chance to selectively excite defined vibrational states in the dimer ion.

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Ion Processes

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In this section we focus our attention on aromatic molecule-noble gas complexes with three van der Waals modes and no strong structural change during the ionization process. We will demonstrate that, in these cases, the newly developed technique of pulsed field threshold

221

ionization leads to the production of state-selected complex ions. 3.2. Pulsedfield

threshold ionization

Delayed pulsed field ionization

is a powerful

0a

0b

acceleration

drift

reflector

Fig. 5. Principle of mass selected pulsed field ionization in a reflectron time-of-flight mass spectrometer. (a) t = 0: molecules are excited by resonance enhanced two-color two-photon excitation. A weak deceleration field leads to the separation of instantaneously ionized molecules (+) and the neutral Rydberg molecules (R). (b) t a 5 ps: a strong electric field is applied. It ionizes the still existing Rydberg molecules and accelerates all ions. The directly ionized molecules are now separated from the newly produced ions and accelerated by a higher potential difference. (c) t M 25~s: due to their larger kinetic energy they penetrate through the reflector, whereas the ions produced by pulsed field ionization are reflected and monitored separately.

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technique when combined with narrow bandwidth laser excitation. It allows the separation of molecules in a narrow energy range close to the ionization threshold from simultaneously excited molecules in lower-lying Rydberg states and from non-energy-selected ions produced by direct ionization above a lower ionization potential. This is probably due to the strongly increasing lifetime of the Rydberg states approaching the ionization energy. The lifetime enlarges due to an increase of the radiative and the non-radiative lifetime, both increasing with n3, where n is the principal quantum number. For states with n > 100, this lifetime exceeds several microseconds in many molecules. Thus, delay times in the microsecond range can be used to field ionize the molecules from their Rydberg states. During this long delay time, the simultaneously-produced, undesired, non-energy-selected ions can be spatially separated from the still-neutral molecules in Rydberg states simply by applying a weak field insufficient to field ionize all of the latter but sufficient to decelerate the ions. First, pulsed field ionization has been used to measure electrons with nearly zero kinetic energy [8]. The technique developed by Reiser et al. [8] and Miiller-Dethlefs and Schlag [40] displays a considerably increased resolution in photoelectron spectroscopy of the electronic ground state ions. Recently, Zhu and Johnson presented a method to combine pulsed field ionization with ion detection [9]. In their technique they separated the threshold ions from ions below this threshold that are responsible for the background in photoionization efficiency spectra by a special technique. This includes three different acceleration regions and a time-of-flight analysis in the mass spectrometer. A similar method was presented by Jouvet et al. [41]. In our recent work we introduced another versatile, easily applicable method for separation of threshold and non-threshold ions. It is based on the energy selection in the reflecting field of a linear reflectron mass spectrometer and provides high mass resolution which is particularly important for cluster investigations [ 18,191.

J. Mass Spectrom. Ion Processes 131 (1994) 211-232

The principle of our method to separate the ions produced by pulsed field ionization (PFI) from directly ionized molecules is shown in Fig. 5. The molecules enter the mass spectrometer with the velocity vj,, of the molecular beam, which depends on the noble gas used (He or Ar). At this time

a

hv, + hv, = 74900 cm-’

C6H6

‘3CC,H,

A

J

l!kdfL__

b

IV] t hv, = 74700 cm-’

, 44

44.5

Time-of-Flight

[ps]

Fig. 6. Time-of-flight mass spectrum of benzene obtained for two different two-photon ionization energies and fixed hut tuned to the 6’ intermediate state. (a) The two-photon energy hv, + hy = 74 900 cm -’ is in resonance with the ionization threshold of the 6’(3/2) ion state (see right arrow in Fig. 9). Two different ion peaks are observed: threshold ions produced by delayed pulsed field ionization according to the method described in Fig. 5 (shaded peaks) and promptly produced ions due to the lower lying adiabatic ionization energy. The same peak pattern is observed for the “light” benzene ‘2CsH6 and the “heavy” ” C’2CsHs isotope. The time separation of threshold ions (shaded) and promptly produced ions is due to the mechanism explained in Fig. 5. Note the high mass resolution which is suRcient to separate all different peaks. (b) Two-photon energy hv, + hy = 74 700 cm-’ is not in resonance with an ionization threshold (see left arrow in Fig. 9). No threshold ions appear in the time-of-flight mass spectrum.

H.J. Neusser and H. KrauselInt. J. Mass Spectrom. Ion Processes 131 (1994) 211-232

(t = 0) a weak electric field of Ei M -0.2 to -0.6Vcm-’ is present (Fig. 5(a)). Its direction is chosen such that the ions instantaneously produced by the laser pulse are decelerated. In the present experiment the pulsed acceleration voltage is delayed by several microseconds after the exciting laser pulse. This is the main difference between the

v, + hv, = 74900 cm”

I I I I I I I t 3 I I3 44

44.5

Time-of-Flight

I t I I k

45

[ps]

Fig. 7. Time-of-flight mass spectra of benzene obtained for a two-photon energy hq + hy = 749OOcn-’ (right arrow in Fig. 9) in resonance with the ionization threshold of the 6’(3/2) ion state. The reflecting voltage of the reflectron mass spectrometer is decreased from (a) to (c). (a) For Urer = 847 V, threshold ions (shaded) and directly produced ions are reflected and monitored. Same situation as in Fig. 6(a). (b) For U,r = 84OV, directly produced ions are reflected with smaller efficiency due to their higher kinetic energy. (c) For &.r = 820 V, the directly produced ions are no longer reflected due to their higher kinetic energy. Only threshold ions with low kinetic energy are reflected and exclusively monitored (see Fig. 5(c). Note that under these conditions the resulting mass spectrum consists of threshold ions (shaded) exclusively without any background from directly produced ions.

223

conventional mass spectrometric operation of the reflectron and the technique described here. After a delay of 3- 100 ps the highly excited neutral molecules in long-lived Rydberg states and the ions originally produced at the same place are spatially separated - by about 1 mm in the case of a 5 ps delay (Fig. 5(b)). At this time a pulsed positive voltage of 1OOOVis applied to the repeller plate. This leads to an electric field of +333 V cm-‘, causing the field ionization of the still existing long lived Rydberg states and the rapid acceleration of all ions. Due to the spatial separation the instantaneously produced ions are accelerated by a higher potential difference than the ions produced by delayed PFI. For a typical spatial separation of 1 mm, this energy difference is about 33 eV. It is sufficient to separate both species within the reflector of the mass spectrometer. The reflecting voltage U,,r is chosen so that only PFI ions are reflected, while the instantaneously produced ions penetrate through the reflector (Fig. 5(c)). In this way the PFI ions are detected after reflection and after they have passed the second drift region. The procedure of separating the directly produced from field ionized ions is demonstrated in Figs. 6 and 7 for the benzene cation. In these spectra a delay time between the laser pulse and the pulsed field ionization of 22~s was used. During this time a permanent field of 0.5 V cm-’ was present to decelerate the directly produced ions. After 22 ,M an electric field of about 500Vcm-’ was turned on in order to ionize the Rydberg states and to accelerate all ions into the mass spectrometer. A reflector voltage Uref = 847 V is high enough to guarantee the detection of all, i.e. directly ionized and field ionized, ions. The reflecting conditions were chosen in such a way that both species are well separated in time. Figure 6(a) shows a part of the mass spectrum in the region between 77 and 79 u, corresponding in flight times between 44 and 45~s. In Fig. 6(a) the wavelength of the second laser was chosen so that Rydberg states converging to the ionization energy of the 6’ (3/2) state are excited, as marked by the right arrow in Fig. 9. In this case, additional ion peaks

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(shaded) with a IOO-ns-longer time of flight than the directly produced ions are observed in the mass spectrum and can be clearly distinguished in the mass spectrum due to the high mass resolution achieved in the reflectron mass spectrometer. Both types of ions are observed for the same excitation energy. Direct ionization corresponds to the adiabatic IE(0’) whereas threshold ions correspond to the 6l(3/2) state. The small mass peak at longer flight times results from the 6% ‘3C’2CsH6 which is present in the natural isotopic benzene sample. For the photon energy hvt = 38 609 cm-’ resonance enhancement of both isotopic species, 12CsH6 and 13C’2CSH6 occurs (see Fig. 10). The adiabatic ionization energies differ only slightly so that threshold ions of both isotopes are observed for the same twophoton energy. The smaller peak at shorter flight times results from the fragmentation of the benzene cation at the four-photon level at the high laser intensity in this experiment. Two additional photons with energy hy are absorbed and lead to the ejection of an H atom from the benzene cation since the total internal energy of the benzene cation after four-photon absorption is more than 89eV. Thus, a rapid dissociation takes place in the ionization region [42,43] and the fragment ion peak appears at the position of the daughter ion. In the mass spectrum of Fig. 6(b) the total twophoton excitation energy was decreased by 200cm-’ (see left arrow in Fig. 9). It is no longer in resonance with long lived Rydberg states close to an ionization potential. For this reason only the directly produced ions corresponding to the 0’ adiabatic ionization energy appear in the mass spectrum, whereas the threshold ion peak has disappeared from the spectrum. For this excitation energy no threshold ions exist since the twophoton energy is not resonant with an ionic state. In Fig. 7 we demonstrate the energy analysis in the reflecting field for suppression of the ions produced by direct ionization. These have a somewhat higher kinetic energy, as demonstrated in Fig. 5. Figure 7 shows a series of mass spectra that have been obtained for different reflection potentials.

J. Mass Specrrom. Ion Processes 131 (1994) 211-232

The exciting two-photon energy is 74 900cm-’ and marked by the right arrow in Fig. 9. For this two-photon energy both directly produced and threshold ions (shaded peaks) are produced. In Fig. 7(a) the reflectron voltage is largest, UIer = 847 V, and both types of ions are reflected and detected by the channel plates. In Fig. 7(b) U,,r is decreased to 840V leading to a smaller signal of the directly produced ions, since only a small part of them is reflected under these conditions. Finally in Fig. 7(c), at Uref = 820 V, the directly produced ions are no longer reflected and do not appear in the mass spectrum, i.e. the observed ions are exclusively produced by pulsed field ionization of long lived Rydberg states. (It should be noted that in the spectra of Fig. 7 the mass resolution is not as high as usual, since the reflectron is operated with incomplete correction so that both ion species are separated in time of flight and can be clearly distinguished from each other.) As a result we obtain ion signals originating exclusively from threshold ions with no background from non-energy-selected directly produced ions. Figure 8 demonstrates the high mass resolution of our experimental setup. Now the reflectron is operated in the complete correction mode. The spectrum displays two sharp PFI peaks which C&h (78 u)

z

.%J CA

E ‘%X,H,

(79 u)

A

I 41

42

Time-of-Flight

[ps]

Fig. 8. Mass spectrum of the threshold ions (shaded) of natural isotopic benzene. The voltages of the reflecting field were optimized to achieve a high mass resolution. Note the large spacing between the two benzene isotopic peaks with one mass unit difference.

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belong to the benzene cation C6Hl(78 u) and the isotope ‘3CC5Hl(79u) resulting from an excitation to the ionic ground state 0’. A mass resolution of about m/Am = 1000 is deduced from the peak widths and the distance between both peaks. In another reflectron apparatus with a longer drift region we were able to realize a mass resolution up to m/Am = 3000 for the threshold ions. The vibrational spectra resulting from a scan of the second laser frequency and the monitoring of the threshold ion current will be discussed in section 3.3. An important difference to the technique of Zhu and Johnson [9] is the high field of some 300 V cm-’ used for delayed field ionization in our experiment. Assuming the well-known field dependence of the ionization threshold [44,45] this field is sufficient to ionize Rydberg states down to lOOcm_’ below threshold. If all Rydberg states within the energy range of 100 cm -’ below threshold survived the delay of several microseconds, the spectral resolution and thus the energy selection would be limited to lOOcm_‘. As will be shown below our achieved energy resolution is a few reciprocal centimeters and thus much better than expected for the high ionizing field. This means that Rydberg states within a narrower energy range are ionized and the lifetime of the lower Rydberg states accessible to field ionization is too short to survive the long delay time of several microseconds. From this result we expect a further improvement of spectral resolution when longer delay times up to 100 ps are used. It should be mentioned that the high ionization field is not principally necessary in our technique, but facilitates the separation of threshold ions from directly produced ions. Up to now, for all investigated molecules, a resolution of several reciprocal centimeters was found and we have not been able to find an example where the high field of 300 V cm-’ led to a lower resolution. Alternatively, a pulsed ionization field comparable to that used in zero kinetic energy electron (ZEKE) experiments (some 1 V cn-‘) [40] can be applied. An additional delayed high field is then used for the acceleration of the so-produced ions.

3.3. Threshold ion spectra

Figure 9 shows a spectrum of the ionic ground state of benzene 6, 2E1,) from zero to lOOOcm_’ excess energy. The total ion current spectrum (middle trace) is compared to a conventional time-of-flight photoelectron (TOF-PE) spectrum (upper trace) and the PFI spectrum (bottom trace). The total ion efficiency curve displays a steplike increase of the ion current whenever a new ionization threshold is reached. The two arrows mark the excitation energy leading to the mass spectra shown in Figs. 6-8. The TOF-PE spectrum has been measured in our laboratory but also previously with a somewhat better resolution by Long et al. [46]. All three experiments have been performed using a two-photon ionization process

1

j’yc

Jo0

t

t I

74400

74600

74800

75000

75200

,

-I!

00

Two-Photon Energy [cm-‘] Fig. 9. Three spectra of the benzene cation CsHt obtained with three different techniques. Top: time-of-flight photoelectron spectrum after ionization of benzene with a two-photon energy of 77210cm-‘. The spectrum reflects the ground state and the lowest vibrational states of the benzene cation. Middle: total ion current as a function of two-photon energy. For each new ionization threshold a step is observed due to the additional ions produced. Bottom: threshold ion (PFI) signal as a function of two-photon energy. Peaks are observed when a new ionization threshold is reached. For two-photon energies not in resonance with an ionization threshold the threshold ion signal is zero (see Fig. 6). Note the increased resolution of this spectrum leading to additional features not resolved in the upper two spectra.

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with the 6l state acting as an intermediate state. The TOF-PE spectrum was recorded in a onecolor two-photon ionization experiment leading to an excess energy of 2655cm-’ in the ion. In the other experiments with ion detection, the wavelength of the second photon was scanned from the adiabatic ionization potential up to lOOOcm_’ excess energy. Obviously, the resolution in the PFI spectrum is much better than in the TOF-PE features are spectrum and new vibrational resolved. Due to the peaked structure and flat baseline of the PFI spectrum it obviously contains more spectral information than the total ion efficiency curve. Several peaks show a previously unresolved splitting and additional peaks with low intensity are observed. The relative integrated peak intensities in the PFI spectrum are similar to that in the TOF-PE spectrum. This means that in the PFI spectrum, as well as in the TOF-PE spectrum, the intensities are mainly determined by the FranckCondon factors of the respective transition. The peak width of 9cm-’ in the PFI spectrum (bottom) is smaller by a factor of 10 than that in

J. Mass Spectrom. Ion Processes 131 (1994) 211-232

the TOF-PE spectrum (top). This still does not represent the experimental resolution but is caused by the rotational structure of the vibronic transition; the relatively broad linewidth (about 0.8cm-i) of the laser providing the first photon leads to the excitation of many rotational states in the 6’ vibronic intermediate state and consequently to many rotational states in the ion. The detailed assignment of all vibrational bands has been discussed in our previous work [lo]. Briefly, the splitting of the 6’(3/2) band is due to a previously unresolved quadratic Jahn-Teller splitting. The mass selectivity of pulsed field threshold ionization with ion detection is demonstrated in Fig. 10. Here the threshold ion current (PFI) spectrum at different masses (78 and 79 u) is shown. The frequency of the first laser is tuned to the maximum of the 6: band of 13C12CSH6present with an abundance of 6% in the natural isotopic mixture. Since the 6; intermediate state spectrum of ‘3C’2CSH,s overlaps with the band of ‘*ChHh, a separation of both spectra solely by resonance enhancement in the intermediate state is not possible. Here, the

C6H6

6’(+3/2)

6’(W)

I

I

I

74600

I

74800

I

75000

Two-Photon

I

75200

I

75400

I

75600

Energy [cm-‘]

(lower trace) in Fig. 10. Vibrational spectra of the ionic ground state of the two benzene isotopes, “CeH6 (upper trace) and “C”C5H6 natural abundance. The frequency vt of the first laser is the same in both spectra. It is in resonance with the maximum of the 6: band of ‘3C’2C5H6 and favors the ionization of this isotope. For the chosen frequency vt , high rotational Jstates of ‘2C6H6 are excited leading to broad peaks in the upper spectrum. Both isotopic spectra can be measured without interference due to the high mass resolution achieved for the threshold ions in the reflectron mass spectrometer (see Fig. 8).

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Ion Processes 131 (1994) 211-232

mass selectivity of ion detection is needed. As shown in Fig. 8, the mass resolution of the reflectron mass spectrometer for the threshold ions is sufficient to separate completely these neighboring isotopic mass peaks in the mass spectrum. In Fig. 10 the frequency of the second laser is scanned across the ionization thresholds. The lower trace represents the threshold ion current at mass 79~ (‘3C’2CsHs) whereas the upper spectrum was obtained for 78~ (12CsHs). The spectrum of 13C12CSH,fresembles the spectrum of “light” benzene (78 u) shown in Fig. 9. However, there are two small differences: (i) The 0’ peak of 13C’2CSHt appears at 74 559 cm-‘, i.e. 4cm-’ higher in energy than the 0’ transition in 12C6H6f. (ii) There is a small peak on the red wing of the 0’ transition of 13C12CSHi which can be tentatively explained by a splitting due to the reduced symmetry that has been theoretically predicted for isotopically substituted benzene cations [47]. The striking difference between these spectra is the broad structure of the vibrational peaks in the “light” benzene (78~) spectrum in Fig. 10 (upper trace) which also differs from that in the spectrum of Fig. 9. This is probably caused by different sets of excited J, K rotational levels in the intermediate state. We have to bear in mind that in Fig. 10 the first laser frequency was scanned to the maximum of the 13C’2CSH6 absorption. This frequency position coincides with the red wings of the rotational contour of the 12C6H6 band containing preferentially high J levels. High J levels in the intermediate state lead to rotational transitions in the vibrational band of the ion which are widely spaced. To summarize, the main result of this section (see Fig. 9) is that molecular ions can be prepared in various defined vibrational states when the second laser frequency is tuned to the respective transition and threshold ions are exclusively observed.

3.4. Cluster ion dissociation After having demonstrated the virtue of pulsed field threshold ionization for studies of molecular

ions, we apply this technique to weakly bound molecular complexes. The frequency of the first laser is now tuned to the intermediate state of the benzene-Ar complex that is produced in the cooled molecular beam. The 6: transition is red-shifted by 21 cm-’ from the corresponding transition in bare benzene. Under these excitation conditions preferentially benzene-Ar complexes are ionized due to the selectivity of the intermediate state. A further selectivity is achieved by the mass selective detection of the threshold ions that is possible in our experiment. Figure 11 shows the PFI spectrum of the

C,H,+*C,H,*

I

3

P

1

16’6’(*3/2)

-w I1

Ar+ I

0"

-z

Ia

’

*,

74400

C,H,- Ar+

6’(*3/2)

-

1,

74600

I

L

74800

I

I

I

I

75000

I

I

II

75200

%

I

I

I

75400

Two-Photon Energy [cm-‘] Fig. 11. Three threshold ion spectra obtained for a molecular beam with benzene seeded in Ar under high pressure. Bottom: threshold ion signal at 78 u for a photon energy hv, in resonance with the 6’ state of C,H,; the vibrational spectrum of the benzene cation is measured. Middle: threshold ion signal at 118 u for a photon energy hu, in resonance with the 6’ intermediate state of the neutral benzene-Ar van der Waals dimer. The resulting spectrum is shifted by 172 cm-’ to the red of the benzene spectrum (lower trace) due to the increased binding energy in the ion. All peaks with a larger energy than the 4’ state disappeared. Top: threshold ion signal at 78~ for a photon energy hv, in resonance with the 6’ intermediate state of the benzene-Ar dimer. The signal at the benzene mass (78~) results from a decay of the complex ion and can be observed at the 16’6’ (&3/2) peak and for peaks with higher energy. For discussions of the dissociation threshold see text.

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benzene-Ar cation (118 u) (middle trace). For comparison, in the lower trace the spectrum of the bare benzene cation (Fig. 8) is given. It was obtained for a frequency of the first laser tuned to the 6; transition of bare benzene. The 0’ transition of the benzene-Ar cation is red-shifted by 172 cm-’ due to the stronger bonding of the benzene-Ar cation compared to the neutral complex in the electronic ground state. This represents the shift of the adiabatic ionization potential after complexation. The ionization energy of benzeneAr is found to be IE = 74 383 f 2 cm-’ in good agreement with recent ZEKE results [48]. On the blue side of the 0’ transition small features indicate the additional excitation of van der Waals modes.

The striking result is the absence of any vibrational structure above the 4l band in the spectrum of the benzene-Ar cation. The position of the split 6’ (3/2) band relative to the origin is the same as in the benzene cation. However, in the benzene cation spectrum the 6’(3/2) mode is the strongest band, while in the benzene-Ar cation the 0’ band is the strongest. From the missing peaks at higher energy we conclude that at excess energies of more than 418 cm-’ (position of the 4’ band) fragmentation of the (benzene-Ar)+ complex occurs. If this conelusion is true, a signal should be observed at the daughter ion mass (i.e., the mass 78 u of benzene). The PFI signal monitored at 78 u for the same frequency of the first photon v1 is given in Fig. 11

@(l/2)-Band

6’(3/2)-Band

O”-Band

68.5

68

6’)

/--(C,H6X

I

40

I

50

I

I

60 70 Time-of-Flight

I

80 [vs]

I

90

I 10

Fig. 12. Three threshold ion mass spectra obtained after excitation of three different final states of the benzene-Ar cation. Bottom: final state is the vibrational ground 0’ state of the ion. Only parent ion peaks (CsHh * Ar+) appear. Middle: final state is the 6’ (3/2) state of the ion (E-,, = 368 cm-‘). In addition to the parent ion peak, weak features around the daughter (CsHc) mass are observed and shown on a magnitied scale in the inset. For explanation see text. Top: final state is the 6’ (l/2) state of the ion (.&r = 677 cm-‘). The parent ion peak has disappeared and a strong daughter ion peak appears. For explanation of the inset see text.

H.J. Neusser and H. Krause/It& J. Mass Spectrom. Ion Processes 131 (1994) 211-232

(upper trace) and compared with the spectrum of (benzene-Ar)+ monitored at mass 118 u measured at the same excitation conditions (middle trace). Obviously, the missing bands in the spectrum observed at the parent ion mass (118 u) appear when monitoring the daughter ion signal at 78 u. The quality of the mass resolution and of the suppression of the ion signal of non-energy-selected directly produced ions that is achieved with the reflectron mass spectrometer is demonstrated in Fig. 12. The lowest trace displays the threshold mass spectrum of the 0’ band. Here the (benzeneAr)+ peak at 118 u dominates the spectrum. The achieved mass resolution is demonstrated in the inset of Fig. 12 with an expanded horizontal scale. Only very weak signals at the time of flight of the benzene monomer of 55.75+, the benzene dimer of 78 ps, and the trimer of 96 ps are seen. These are produced by direct ionization. Basically the same situation is observed for the 6’(3/2) band with the benzene-Ar peak representing the strongest signal (middle trace). Finally, for the highest excited 6’ (l/2) ion state at an excess energy of 677cm-’ the C6H6 - Ar+ peak disappears and instead a strong benzene peak grows in. Since nothing was changed in the intermediate state these benzene cations are clearly the product of the (benzene-Ar)+ dissociation. The additional small features at somewhat (0.2 and 0.6 ps) longer flight times (see inset) result from a slow metastable dissociation of non-energy-selected benzene-Ar and benzene dimer ions, respectively. Thus we conclude that the dissociation of C6H6 - Ar complexes in long-lived high Rydberg states occurs before they are ionized by the pulsed field. This implies that the dissociation process does not disturb the Rydberg electron. Since the dissociation process is the same for the electron in a high Rydberg orbit or removed from the core, this clearly demonstrates that the benzene-Ar cation dissociates on a time scale of less than 100 ps by evaporation of the Ar atom at an internal energy smaller than 629cm-’ (position of the 16l6’(3/2) band). In conclusion, the first observed peak in the daughter ion spectrum at 629 cm-’ yields an upper limit for the dissociation energy of the benzene-Ar cation, i.e.,

229

E. < 629cm-’ (78meV), (16l6’(3/2) level). In our recent publication we found daughter ions already at the smaller internal energy of the 6’ (3/2) peak [lo]. At present we cannot exclude the possibility that this signal originated from a three-photon excitation process. To be on the safe side we take the energy of the 16’6’ (3/2) band as an upper limit. Experiments on this point will be discussed in more detail in a separate publication [49]. From the 172 cm-’ redshift of the ionization energy of benzene-Ar, an upper limit of Do < 457cm-’ (56meV) results for the dissociation energy of the neutral benzene-Ar complex using the relation Do = E. - AIE. These upper limits can be reduced considerably by comparing the results of the benzene-Ar complex with those of benzene-Kr or other benzene-noble gas clusters, as will be presented in a forthcoming paper [49]. It is interesting to compare the results for the upper limits of the dissociation energies of the neutral and ionic benzene-Ar complex with the results for heterogeneous benzene-molecule dimers in Table 1. In the ionic benzene-Ar complex, charge transfer resonance interaction is not expected to contribute to the binding energy. The ionization energy of Ar (15.76eV [50]) is much higher than that of benzene (9.243eV [48,51]). Thus, the charge is located on the benzene part of the complex. This is the same situation as e.g. in the benzene-cyclohexane dimer where the charge is also located on the benzene side of the complex. The smaller dissociation energy of the ionic benzeneAr complex is then qualitatively explained by the smaller polarizability (1.64 A3 [27]) of Ar compared to cyclohexane (11.0 A3[27]). The same argument also holds for the neutral benzene-Ar complex: the dissociation energy of benzene-Ar is expected to be smaller than the dissociation energy of the other benzene-molecule dimers listed in Table 1 because of the smaller polarizability of Ar. The upper limit for the dissociation energy of the neutral benzene-Ar complex found in this work from the PFI spectra in Fig. 11 is in formal agreement with the binding energies found with different theoretical methods. Force field calculations

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yielded a value of 393cm-’ [52], and recent ab initio calculations a value of 380cn-’ [53]. However, we have to bear in mind that the experimental upper limit of this work represents a value that is most likely above the real value. The reason for this is the sparse vibrational level structure of the benzene-Ar cation around the dissociation threshold. A dissociation cannot be observed for the 4l level at 418 cm-’ excess energy but it is observed at the 16’ 6’ (3/2) level at 629 cm-‘. For a critical comparison with theoretical predictions the gap between the highest non-dissociating level and the lowest dissociating level should be decreased. This will be shown in a separate publication on hand of measurements of benzene-Kr dissociation threshold [49]. 4. Summary and conclusion In this work, various new aspects of time-offlight mass spectrometry are discussed which are important for the investigation of weakly bound van der Waals clusters. Two different methods of studying the energetics of molecular clusters have been presented. Both techniques employ a cooled supersonic beam combined with a time-of-flight mass spectrometer. The special instrument used here is a linear reflectron mass spectrometer with a molecular beam collinear to the ion flight paths. For ionization, resonance-enhanced two-photon ionization is particularly suitable as the resonance enhancement in the first absorption step assists the selection of a single cluster species from the mixture of clusters produced in the molecular beam. For the determination of their dissociation threshold the investigated ions must be produced with defined internal energy distributions. In breakdown measurements there exists a sharp upper limit for this energy distribution given by the total photon energy. In this limiting case the excess energy above the adiabatic ionization energy is present as internal energy of the ion, i.e. the kinetic energy of the electron is zero. When the decay of an ion is observed as a function of this photon energy the fragment ion intensity breaks down for a

photon energy below threshold. The resulting “breakdown graphs” yield appearance energies for the fragments. This breakdown is close to the dissociation threshold with a negligible kinetic shift when slow metastable dissociation is observed. With this technique the dissociation energies of several molecular clusters containing an aromatic molecule as a chromophor have been obtained. The second method permits the production of cluster ions with sharply defined internal energy rather than a distribution of energy. This is a novel result for clusters achieved with a special technique based on pulsed field ionization of long-lived Rydberg states close to a higher ionization threshold. Threshold ions appear when the increasing two-photon energy reaches a new ionization threshold. The separation of the energyselected threshold ions from non-energy-selected ones is performed by the energy-analyzing properties of the reflecting field in a reflectron mass spectrometer [lo]. As a result the threshold ion spectrum contains sharp peaks at the different vibrational levels of the ions on a flat baseline rather than steps as in the conventional ionization efficiency curves. Although the separation of ions can be achieved with other techniques [9,41], the energy analysis in a reflectron time-of-flight mass spectrometer provides in addition a high mass resolution due to its flight time correction properties. This is particularly important for cluster investigations (as their mass spectra contain a great number of peaks and the spacing between neighboring peaks is small). Several features of the mass-selective pulsed field ionization technique used in this work are described. First experiments with this powerful technique demonstrate that the production of state-selected benzene-noble gas dimer ions is possible and that their decay can be observed. Future applications will include other dimer ions, larger ionic clusters and spectroscopic studies of their ground states. Good candidates for these investigations are clusters that do not undergo large structural changes after ionization and have a small number of van der Waals modes. This behavior is expected for systems

H.J. Neusser and H. KrauselInt.

J. Mass Spectrom.

Ion Processes

with weak contributions from charge transfer resonance interaction in the ionic complex, such as clusters composed of constituents with strongly differing ionization energy. For the spectral resolution demonstrated in this work the investigation of van der Waals modes, even in weakly bound ionic complexes, is possible [54]. This is important information complementary to the spectral information available for the respective neutral systems. At present, the spectral resolution is limited to about 1.6cm-’ for delay times of some 20~s [%I. One expects that the lifetimes of higher Rydberg states are even longer. Thus, experiments with longer delay times of some 100 ps at low densities and small electric fields will further increase the spectral resolution and suppress the linewidth below the 1 cm-’ level. First experiments in our laboratory have indeed shown that a sufficient number of Rydberg states survives a 200~s delay and a threshold ion signal can be detected [49]. This high spectral resolution, together with the high mass resolution of the reflectron mass spectrometer, makes this technique valuable for the spectroscopic investigation of not only clusters, but also radicals or transient species that are present at low concentrations within a molecular beam sample. Not only spectroscopic but also kinetic investigations of ions become accessible with this new technique. When a metastable decay of the energy-selected threshold ions occurs in the drift region of the reflectron time-of-flight mass spectrometer, the resulting drift peak can be analyzed and decay constants can be deduced. In this way, unimolecular kinetic models and the validity of statistical assumptions can be experimentally tested. This is of particular importance for molecular clusters since, in these systems, the intramolecular energy redistribution process is expected to be different due to the existence of two types of vibrations, intra- and intermolecular, with strongly differing frequencies. In conclusion, we have shown that recent developments in pulsed field ionization demonstrate the importance of time-of-flight mass spec-

131 (1994) 211-232

231

trometry for the investigation of the spectroscopy and decay kinetics of molecular and particularly cluster ions.

Acknowledgments

The authors thank Professor Schlag for the invitation to contribute to this special issue. They are grateful to Dr. Alice Smith for careful reading of the manuscript. Financial support from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged.

References

4 5 6 1 8 9

10 11 12 13 14 15 16 17 18 19

R.G. Cooks, J.H. Beynon, R.M. Caprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. W. Forst, Theory of Unimolecular Reactions, Academic Press, New York, 1973. W.J. Chesnavich and M.T. Bowers, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Vol. 1, Academic Press, New York, 1919. C. Lifshitz, Int. J. Mass Spectrom. Ion Processes, 1IS/119 (1992) 315. H.J. Neusser, J. Phys. Chem., 93 (1989) 3897. B. Brehm and E. von Puttkamer, Z. Naturforsch. Teil A, 22 (1967) 8. T. Bear, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Vol. 1, Academic Press, New York, 1979. G. Reiser, W. Habenicht, K. Mtiller-Dethlefs and E.W. Schlag, Chem. Phys. Lett., 152 (1988) 119. L. Zhu and P. Johnson, J. Chem Phys., 94 (1991) 5769. H. Krause and H.J. Neusser, J. Chem. Phys., 97 (1992) 5923. U. Boesl, H.J. Neusser and E.W. Schlag, Z. Naturforsch. Teil A, 33 (1978) 1546. L. Zandee, R.B. Bernstein and D.A. Lichtin, J. Chem. Phys., 69 (1978) 3247. E.W. Schlag and H.J. Neusser, Act. Chem. Res., 16 (1983) 355. H.J. Neusser, Int. J. Mass Spectrom. Ion Processes, 79 (1987) 141. C. Lifshitz and N. Ohnmichi, J. Phys. Chem., 93 (1989) 6329. M.R. Nimlos, M.A. Young, E.R. Bernstein and D.F. Kelley, J. Chem. Phys., 91 (1989) 5268. B. Ernstberger, H. Krause, A. Kiermeier and H.J. Neusser, J. Chem. Phys., 92 (1990) 5285. A. Kiermeier, B. Emstberger, H.J. Neusser and E.W. Schlag, J. Phys. Chem., 92 (1988) 3785. H. Kilhlewind, H.J. Neusser and E.W. Schlag, Int. J. Mass Spectrom. Ion Phys., 51 (1983) 255.

232

20

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

H.J. Newer

and H. Krause/h.

H.L. Selzle, H.J. Neusser, B. Emstberger, H. Krause and E.W. S&lag, J. Phys. Chem., 93 (1989) 7535. P.M. Dehmer, J. Chem. Phys.,76 (1982) 1263. E. Riihl, B. Brutschy and H. Baumglrtel, Chem. Phys. Lett., 157 (1989) 379. J.R. Grover, G. Hagenow and E.A. Walters, J. Chem. Phys., 97 (1992) 628. K.H. Fung, W.E. Henke, T.R. Hays, H.L. Selzle and E.W. Schlag, J. Phys. Chem., 85 (1981) 3560. B. Ernstberger, H. Krause and H.J. Neusser, Z. Phys. D, 20 (1991) 189. W. Scherzer, 0. KrHtzschmar, H.L. Selzle and E.W. Schlag, Z. Naturforsch. Teil A, 47 (1992) 1248. R.C. Weast and M.J. Astle (Eds.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1979. A.L. McClellan, Tables of Experimental Dipole Moments, Rahara Enterprises, El Cerrito, 1974. H. Krause, B. Emstberger and H.J. Neusser, Chem. Phys. Lett., 184 (1991) 411. B.W. van de Waal, Chem. Phys. Lett., 123 (1986) 69. A. de Meijere and F. Huisken, J. Chem. Phys., 92 (1990) 5826. P. C&sky, H.L. Selzle and E.W. Schlag, Chem. Phys., 125 (1988) 165. P. Hobza, H.L. Selzle and E.W. Schlag, J. Chem. Phys., 93 (1990) 5893. 0. Kratzschmar, H.L. Selzle and E.W. Schlag, in preparation. M. Meot-Ner, P. Hamlet, E.P. Hunter and F.H. Field, J. Am. Chem. Sot., 100 (1978) 5466. B. Badger and B. Brocklehurst, Nature, 219 (1968) 263.

J. Mass Spectrom. Ion Processes 131 (1994) 211-232

37 F.H. Field, P. Hamlet and W.F. Libby, J. Am. Chem. Sot., 91 (1969) 2839. 38 E.G. Jones, A.K. Bhattacharya and T.O. Tieman, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 147. 39 K. Hiraoka, S. Fujimaki, K. Aruga and S. Yamabe, J. Chem. Phys., 95 (1991) 8413. 40 K. Mtiller-Dethlefs and E.W. Schlag, Ann. Rev. Phys. Chem., 42 (1991) 109. 41 C. Jouvet, C. Dedonder-Lardeux. S. Martrenchard-Barra and D. Solgadi, Chem. Phys. Lett., 198 (1992) 419. 42 H. Ktihlewind, A. Kiermeier and H.J. Neusser, J. Chem. Phys., 85 (1986) 4427. 43 H. Ktihlewind, A. Kiermeier, H.J. Neusser and E.W. Schlag, J. Chem. Phys., 87 (1987) 6488. 44 W.E. Cooke and T.F. Gallagher, Phys. Rev. A, 17 (1978) 1226. 45 K.H. Fung, H.L. Selzle and E.W. Schlag, Z. Naturforsch. Teil A. 36 (1981) 1257. 46 S.R. Long, J.T. Meek and J.P. Reilly, J. Chem. Phys., 79 (1983) 3206. 47 J. Eiding and W. Domcke, Chem. Phys., 163 (1992) 133. 48 L.A. Chewter, K. Milller-Dethlefs and E.W. Schlag, Chem. Phys. Lett., 135 (1987) 219. 49 H. Krause and H.J. Neusser, J. Chem. Phys., in press. 50 S.G. Lias, J.E. Bartmess, J.F. Leibman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 17 (suppl. 1) (1988). 51 S.G. Grubb, R.L. Whetten, A.C. Albrecht and E.R. Grant, Chem. Phys. Lett., 108 (1984) 420. 52 N.B. Horin, U. Even and J. Jortner, J. Chem. Phys., 91 (1989) 331. 53 P. Hobza, H.L. Selzle and E.W. Schlag, J. Chem. Phys., 95 (1991) 391.