ELSEVIER

24 October 1997

Chemical Physics Letters 278 (1997) 63-70

CHEMICAL PHYSICS LETTERS

The photoionization of ArHC1

Yue Li ", Xiu-yan Wang a, Xiao-guang Zhang a, Lian-bin Li a, Nan-quan Lou a, Liu-si Sheng b, Yun-wu Zhang b

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute qf Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

b National Synchrotron Radiation Laboratory, Uniuersity of Science and Technology of China, HeJei, China

Received 19 March 1997; in final form 3 September 1997

Abstract

The photoionization of the van der Waals cluster ArHC1 in a molecular beam has been investigated, using a synchrotron radiation light source and a photoionization mass spectrometer. The concentration of ArHCI is approximately given by the equation ce(ArHCl)% = 1.79 X 10 - s Po 15, where P0 is the stagnation pressure in Pa. The photoionization efficiency curve of ArHC1 was obtained and the appearance potential of ArHC1 + was determined to be 12.52_+ 0.03 eV. The bond dissociation energy D o of ArHC1 + is deduced to be 0.24_+ 0.04 eV. The theoretical calculation using Gaussian-94w indicates that ArHCI + has a linear conformation like ArHC1. The auto-ionization mechanism of ArHC1 and charge transfer-vibrational predissociation processes are discussed. © 1997 Elsevier Science B.V.

1. Introduct ion

The photoionization of cluster beams has been one of the major sources of information on the properties of ionic as well as neutral clusters [1]. One-photon ionization by using synchrotron radia- tion (SR) as a light source is often used to study the size dependence of ionization potentials (IP) and fragmentation appearance potentials (FAP) of clus- ters. Moreover, one can acquire fundamental data, such as formation enthalpies, dissociation energies and proton affinities of the clusters, etc. Among these, the photoionization investigations of van der Waals (vdW) clusters are attractive. Recently they have been of considerable interest both to experi- mentalists and theoreticians [2,3]. The vdW interac- tion is generally so weak that the molecular subunits retain their individuality, allowing one to selectively excite or ionize them. Cluster ions, which can be

obtained by photoionization of neutral clusters, are often taken as models for transition states in ion- molecule reactions in the sense that their fragmenta- tion represents the 'half-collision' analogues to the latter [4]. From the point of view of chemical dynam- ics, photoionization and photoexcitation in the VUV region of dimers and clusters can offer a direct and general route for the preparation of collision com- plexes and examination of the decomposition of these complexes as a function of internal excitation; hence it is a valuable tool for investigating ion- molecule reactions and high-Rydberg-state chemistry [2]. The investigations of inner relaxation processes after photoionization of clusters, such as proton transfer, charge transfer and chemical reactions, etc., are meaningful for understanding molecular dynam- ics.

Since a calculation for the ArHC1 cluster is rela- tively easy, and Ar and HC1 are also familiar, ArHCI

0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0009-261 4 (97 )00999 - 8

64 Y. Li et aL / Chemical Physics Letters 278 (1997) 63-70

is an ideal system to investigate the photochemistry of clusters. Many investigations have been per- formed on the structure of the system in both theoret- ical and experimental work. Molecular beam electric resonance spectroscopy has shown that the cluster probably has a linear equilibrium geometry, but bends substantially (41.5 ° ) in the ground state [5-7]. The isotropic well depth has been determined to be 133 c m - 1 by molecular beam scattering experiments [8]. Potential energy surfaces for this system have been proposed by Hutson and Howard by means of a multiproperty fitting of microwave spectra, far-in- frared HC1 pressure broadening coefficients, second virial coefficients and molecular beam scattering data [9]. They show that ArHC1 is linear and the H atom lies between the Ar and C1 atoms with an equilib- rium well depth of 180 cm 1. High-resolution spec- troscopic techniques have been used to probe the fundamental vibrations and rotations of ArHC1 and the agreement with the predictions from the Hutson and Howard M5 potential is good [10-12]. In a series of recent articles [13-15], the photodissocia- tion dynamics of HC1 in the clusters ArHC1 have been studied by Garcia-Vela et al. to explore the cluster size effect.

Little work has been done on the photoionization of ArHC1. This Letter presents some new results obtained by using a synchrotron radiation light source and a photoionization mass spectrometer, such as the relative concentration of the cluster and the dissocia- tion energy of the cluster ion, etc., and the auto-ioni- zation mechanism is also discussed. When Ar in ArHCI is ionized, the charge transfer-vibrational predissociation processes have been proposed. The experiments were complemented by ab initio calcula- tions using the Gaussian-94w program package [16].

2. Experimental

Our experiment was undertaken in the Photo- chemistry Station of NSRL. The experimental ar- rangement has been described in detail previously [17]. Briefly, the apparatus consisted of four sec- tions: a vacuum ultraviolet (VUV) monochromator, a supersonic molecular beam, a photoionization cham- ber and a data acquisition and control system. A VUV light beam from a storage ring was passed

through a Seya-Namioka VUV monochromator where three gratings (2400, 1200, 600 l ines /mm) were installed with a wavelength range of 35-600 nm and a resolution of 0.1 nm. The wavelength was calibrated with the first ionization potential of Ar or Ne and the error was less than _+ 1.0 A. The scan of the grating was achieved by a stepper motor that was controlled by a computer.

A gas mixture containing Ar and HC1 (the mixing ratio of Ar to HC1 was 5%) passed through a 50 Ixm diameter nozzle and expanded into a beam source chamber. By a 1 mm diameter skimmer between the beam source chamber and an ionization chamber, a straight continuous molecular beam was formed. In the experiment, the typical background pressure in the ionization chamber was 5 × 10 -5 Pa. In the chamber the molecular beam was vertically inter- sected by the light beam. A quadrupole mass spec- trometer or a time of flight (TOF) mass spectrometer was perpendicular to both beams. The signals de- tected by MCP were amplified, discriminated and stored on a computer. The wavelength scan and the data acquisition were controlled by the computer.

3. Results and discussion

3.1. Concentration of the ArHC1 cluster

Fig. 1 shows part of a cluster ion mass spectrum obtained in this experiment. The intense peaks corre-

400

0 70 72

300

200

IO0

Ar2 ÷

(HCI)~*

A rHCI ÷

74 76 78 80 82 M a s s number

Fig. 1. Photoionization mass spectra of Ar /HC1 clusters produced in supersonic expansion of 5% HC1 in Ar from a source pressure of 1 atm at the wavelength 760 A.

Y. Li et al. / Chemical Physics Letters 278 (1997) 63-70 6 5

spond to (HC1)~-(72), ArHCl+(76) and Ar~-(80), respectively. Mass numbers 74 and 78 are peaks of the isotopes of (HC1)~- and ArHCI+(3VC1 24.2%).

The concentrations of clusters in a supersonic beam are relative to a stagnation pressure, a nozzle diameter, the stagnation nozzle temperature, etc. [2]. It is difficult to measure precisely a cluster concen- tration in a molecular beam or in a cell. By means of the 'soft ionization' method with synchrotron radia- tion light, the approximate concentration of clusters could be obtained in this experiment. The cluster ion counts, which are measured in the photoionization mass spectrum method (PIMS), are directly propor- tional to the concentration of the neutral cluster, the light intensity and the photoionization cross-section. We selected a photon energy (970 A) a little higher than the ionization potential of HC1 and neglected the dissociation of ArHC1 +. Moreover, the photoion- ization cross-section of ArHC1 at 970 A is assumed to be approximately equal to that of HCI (see Section 3). Thus, the counts ratio of ArHC1 + to HCI + detected is approximately equal to the concentration of ArHC1 relative to HC1 in the beam. Similar meth- ods of the measurement of relative concentrations of clusters have been reported [18,19]. The variation of the concentration of ArHC1 relative to HC1 at 970 is plotted against the nozzle stagnation pressure in Fig. 2.

In a supersonic expansion, dimers are usually found to be proportional to (Podo) ", where P0 is the nozzle stagnation pressure, d o is the nozzle diameter

-~ ..-'6

Q 0.81- .."

~ J ....

0.6 .

0 .4 "

0.2 o . . . - o" . , . . -

O0 5 10 15

P000'Pa)

20

Fig. 2. Variation of the concentration of ArHCI relative to HCI versus nozzle stagnation pressure at a nozzle temperature of 298 K. o, Experimental point for ArHC1 obtained at 970 A; dotted line, curve fitted in o~(ArHCI)%= 1.79× 10 8 poLS, where P0 (Pa) is the nozzle stagnation pressure.

and the index number n has a value of approxi- mately 2 [2]. In this experiment, d o was fixed at 0.005 cm, so the relative concentration a(ArHCI)% can be described as In c~ = n In P0 + const., in which n and the constant const, can be determined by using a least-square fitting to the data. The fitting shows that the relative concentration of ArHC1 can be approximately given by the equation

a ( A r H C I ) % = 1.79 × 10 -8 p~.5, (1)

where P0 is in Pa. The fitted curve is also shown in Fig. 2. In the above equation, the index number n is 1.5 which is the same as that for Ar 2, but the coefficient is much larger than that of Ar 2 [18]. This indicates that the ArHC1 cluster is more easily formed than Ar 2, probably because of the stronger interac- tion between the polar HC1 molecule and Ar than between Ar and Ar.

Fig. 2 shows that ArHC1 is only 0.6% of the HC1 monomer when the stagnation pressure is 1 atm. Increasing the stagnation pressure could effectively increase the concentration of a cluster, but it could also make the background vacuum worse. Thus, during the experiment, the nozzle stagnation pressure did not exceed 3 arm.

In these experimental conditions, any larger clus- ters ions were undetectable, such as Ar2HC1 +, Ar(HC1)~, etc. It was estimated that the intensity of the spectral peaks of the larger clusters was two orders of magnitude lower than that of ArHC1 +. Thus, we assume that the peak of mass 76 in the spectra was produced mainly by the photoionization of ArHC1 and the effects of ionization-dissociation or dissociation-ionization of the larger clusters could be neglected.

3.2. Dissociation energy of ArHC1 +

Much theoretical and experimental work on neu- tral and ionic van der Waals clusters has been per- formed [20-23]. The dissociation energies of the clusters are important for understanding the proper- ties and dynamics of the clusters. In this work, similar photoionization efficiency (PIE) curves of ArHC1 were obtained by using both a quadrupole mass spectrometer and a TOF mass spectrometer; the appearance potential (AP) and dissociation energy of

3-

Y. Li et al. / Chemical Physics Letters 278 (1997) 63-70

==

v

66

.o0

Fig. 3. The photoionization efficiency curve of ArHCI in the wavelength range from 700 to 1000 ,~. (Thin line: the photoion- ization efficiency curve of HC1; two curves are normalized to their maxima in the wavelength range from 950 to 1000 ,~.)

ArHC1 + could thus be determined. A typical PIE curve is shown in Fig. 3. It was estimated that the intensity of the second-order light in the system was two orders of magnitude lower than the principal

light in the wavelength range 700-1000 i . Thus, its effects on the above measurement were so small that it could be neglected. The AP of ArHC1 ÷ was determined to be 12.52 + 0.03 eV, which was red- shifted 0.22 eV relative to that of HC1. Assuming that the AP is identical to the adiabatic ionization potential (IP), the ArHC1 ÷ bond dissociation energy can be calculated using the relation

D0(ArHCI +) = IP(HCI) + D0(ArHC1 )

- IP(ArHC1). (2)

The ionization potential of HC1 was determined to be 12.74 _+ 0.01 eV, which is consistent with the results of other experiments [24]. Using the above results and 180 cm -1 [8] for the dissociation energy of ArHC1, the dissociation energy D o of the ground state of ArHC1 + is deduced to be 0.24 ± 0.04 eV. It is significantly less than the dissociation energies of the homonuclear dimers Ar£(1.269 _+ 0.017 eV) [25] and (HC1)~(0.87 _+ 0.09 eV) [2].

Table 1 Geometry parameters, energies, frequencies, dissociation energies and atomic charges of the most stable conformations of ArHC1 and ArHCI ÷ at different levels of computation

Method R(Ar -H) / ,~ R(H-C1)/~, E/hartree Freq . /cm I (unscaled) De/eV Atomic charge/e

ArHCI H F / 6 - 31G ~ 3.512 1.2662

MP2/6-31G * 2.92 1.2801

MP2/6-31G* * 2.8605 1.2689

QCISD/6-31G * * 2.9024 1.2724

ArHCI + HF/6-31G* 2.3808 1.301

MP2/6-31G" 2.2082 1.3252

MP2/6-31G* * 2.1335 1.3219

QCISD/6-31 G* * 2.1843 1.3204

-986.8338 14, 37, 37, 3187 0.002 Ar 0.001 H 0.242 C1 -0 .243

-987.1039 36, 100, 100, 3051 0.013 Ar 0.004 H 0.239 C1 -0 .243

-987.1171 41, 90, 90, 3125 0.015 Ar 0.005 H 0.185 C1 -0 .190

-987.1455 38, 105, 105, 3073 0.013 Ar 0.006 H 0.164 C1 -0 .170

-986.4108 86, 253, 270, 2817 0.085 Ar 0.043 H 0.337 C1 0.620

-986.6590 114, 353, 370, 2572 0.153 Ar 0.063 H 0.320 C1 0.616

-986.6712 113, 386, 408, 2556 0.171 Ar 0.073 H 0.252 C1 0.675

-986.7029 109, 356, 380, 2592 0.156 Ar 0.075 H 0.233 CI 0.691

E L i et al. / Chemical Physics Letters 278 (1997) 63- 70 67

We have performed ab initio calculations on ArHC1 using Gaussian-94w [16]. Geometry optimiza- tions have been performed at the HF, MP2 and QCISD levels of theory applying the 6-31G* and 6-31G* * standard basis sets without any symmetry restrictions. At all stationary points the frequencies have been calculated to check whether the stationary points correspond to stable conformations but not transition states. At different levels of theory, the dissociation energies D e of ArHCI ( ~ Ar + HCI) and ArHCI + ( ~ Ar + HCI +) were also calculated. The geometry parameters, the energies, the frequen- cies, the dissociation energies D e and the atomic charges are listed in Table 1. The results indicate that the most stable conformation of ArHC1 is a linear equilibrium geometry as obtained by Hutson et al. [9]. The results indicate that the most stable ionic ArHC1 + also has a linear conformation. Relative to the neutral ArHCI, the bond lengths R(Ar-H) and R(H-C1) have changed considerably. At the HF level, the dissociation energy D e of ArHC1 + is only 0.085 eV, which is much smaller than the experi- mental value. The post SCF calculations of MP2, QCISD including the correlation of electrons im- prove the results, certain degree and they show that the dissociation energy of ArHCI + is about 0.16 eV. It is estimated that more accurate model chemistries and larger basis sets are necessary to predict the energetics of the system. When comparing the theo- retical results with experimental values, it must be recognized that the calculated bond energies repre- sent the energy of the potential minimum D e, whereas the experimental ones are for the lowest vibrational energy level D o . The results differ by the zero-point energy of the multidimensional van der Waals poten- tial.

3.3. Photoionization processes of ArHC1

HC1 has the ground state electron configuration KL(3s~r)2(3p~r)Z(3p'rr)4 (J~+). When it is ionized, it loses a non-bonding electron and becomes the HCI + (21-]) ion. The distance between the atoms changes little (R(HC1) = 1.275 A, R(HC1 +) = 1.315 A) [26]. The Ar atom has the same number of electrons as the HCI molecule, but has a much higher ionization potential (IP(Ar)= 15.76 eV). Table 1 shows that the H-C1 distance in ArHCI does not

differ much from that of the HC1 molecule. From the dissociation energies and the atomic charges calcu- lated, it can be seen that for the neutral ArHC1 cluster, the interaction force between Ar and HCI is weak. But for ArHC1 +, relative to the neutral cluster, its subunits have a much stronger interaction force which can be mainly attributed to electrostatic and charge transfer resonance interactions strongly en- hanced after the cluster is ionized [23]. A large difference in the ionization energies leads to a local- ization of the charge at the component with the lower ionization energy and thus to a small charge transfer resonance interaction. Therefore, the in- crease of the interaction between two subunits of ArHC1 ÷ is expected to result dominantly from the stronger electrostatic interaction.

The PIE curves of HCI and Ar are shown in Fig. 4. The absolute photoionization cross-sections of HCI have been reported in some theoretical and experimental papers [27-31]. However, the values proposed are very different. The photoionization cross-sections of Ar are approximately 36 mb in the wavelength range 700-780 A [32]. In this experi- ment, Ar was taken as a standard to calibrate the photoionization cross-section of HC1 which was about 40% of that of Ar at 770 A. Thus, at 770 A, the absolute photoionization cross-section of HC1 is esti- mated to be about 15 mb, which is much smaller than the value reported.

At wavelengths longer than 787 A, the photoion- ization cross-section of Ar is nearly zero. However, at 787 ,~, the photoionization cross-section increases suddenly. The peaks in the wavelength range 700-

"5 ==

o

70

6O

5O

40

3O

20

10

0 700 750 800 850 900 950 1,000

Wav~jth ( ~ n )

Fig. 4. The photoionization efficiency curves of Ar, HCI and the photoionization efficiency curve of ArHC1 expected (dotted line).

68 E L i et al. / Chemical Physics Letters" 278 (1997) 63 70

1000 A in the photoionization spectra of HC1 have been analyzed [27,28] and their results indicate that the rich structure results from both direct ionization and auto-ionization after HC1 is excited to high-Ryd- berg states by photons.

Fig. 3 shows that when the wavelength is longer than 815 A (shown with an arrow), the PIE curve of ArHC1 has the same character as that of the HCI molecule. It embodies the specific property of the photoionization of HCI. Therefore, HCI in the ArHC1 cluster should have a similar photoionization mecha- nism to the HC1 molecule, while the photoionization cross-sections of Ar in ArHC1 are small and Ar does not affect the ionization of HCI significantly. Namely, when photons excite HC1 in ArHC1 to the high-Ryd- berg states, auto-ionization occurs and vibrationally excited ArHC1 + is produced. Strong vibrational exci- tation can make the ion dissociate. However, in our wavelength range, the PIE curve of ArHC1 gives no evidence that the dissociation of ArHC1 + has taken place. Therefore, it is deduced that ArHCI + should be weakly excited, and the greater part of the excess energy should be taken away from the system by photoelectrons, so that ArHC1 + can exist stabl),.

When the wavelength is longer than 815 A, the PIE curve of ArHC1 is determined by the ionization of HC1. Thus, it should be reasonable to assume that ArHC1 has the same photoionization cross-sections as HC1 at these wavelengths. The PIE curves of ArHC1 and HCI are shown in Fig. 3 and their relative intensity is normalized to their maxima in the wave-

o

length range from 950 to 1000 A, where HC1 begins to be ionized. Due to the vdW interaction between Ar and HC1, the ionization potential of Ar in ArHC1 should be red-shifted, and auto-ionization that is analogous to Ar2o [ 18] should occur. However, at 815 A (but not 787 A), when Ar in ArHC1 is excited to high-Rydberg states, it may have two auto-ionization processes, unlike homogeneous clusters - - one cor- responding to Ar+HCI and the other to ArHCl +. Thus, it can be expected that when Ar begins to be ionized, the photoionization cross-sections of ArHC1 should increase. Fig. 3 shows that at wavelengths shorter than 815 A, the photoionization cross-section of ArHC1 increases. It implies that the above mecha- nism is reasonable.

In the photoionization investigations of homoge- nous clusters, Ng et al. assumed the photoionization

cross-section of a dimer to be twice that of the corresponding monomer [2,18]. Similarly, if the pho- toionization cross-section of ArHC1 is presumed to be approximately the sum of those of Ar and HC1, then the expected PIE curve can be obtained as the dotted line shown in Fig. 4. Namely, for this kind of weak interaction system like the ArHC1 cluster, two different ionization threshold values should exist in the PIE curve [3] - - one corresponding to the ioniza- tion of Ar and the second to the ionization of HC1. However, the measured PIE curve is very different, in which two ionization threshold values do not appear to exist and the photoionization cross-sections of ArHCI are much lower than those expected. Thus, it is estimated that the photoionization mechanism of a weak interaction dimer is complicated in the short wavelength region where high-Rydberg states of a subunit can be excited and the interaction between the two subunits becomes strong. The electron clouds of the two subunits will have mixed considerably, causing the obvious ionization peaks of Ar in ArHC1 to be lost.

In the above auto-ionization process, Ar+HC1 should be unstable and the following charge trans- fer-dissociation processes may take place:

Ar+HC1 ~ ArHC1 +* (X), (3)

Ar+HC1 ~ Ar + HC1 +* (X) (4)

(X and v~ indicate the electronic ground state and a vibrationally excited state, respectively)

The ionization potential of Ar is about 3 eV higher than that of HCI. After Ar is ionized, Ar+HCI becomes an unstable 'superexcited' system and the Ar + ion, which has high electronegativity, can gain an electron from the CI atom by a fast electron-hop- ping, releasing about 3 eV of energy. Since both neutral and ionic ArHC1 are linear, the energy should be released into the vibrational modes of the cluster and cause intense vibrational excitation of ArHC1 +. In a vibration period of Ar-H, the weaker bond can be broken and ArHCI + should be dissociated. Charge transfer is so fast that the energy resonance domi- nates the process. The released energy will be princi- pally in the vibrational modes of the H-CI bond, which causes high vibrational excitation of the prod- uct HCl+. The above near-resonance intramolecular

Y. Li et al. / Chemical Physics Letters 278 (1997) 63-70 69

charge transfer and vibrational predissociation pro- cesses have been suggested in some studies of pho- toionization of vdW clusters [3,33]. However, so far, convincing arguments are still deficient. The lumi- nescent charge-transfer reaction between Ar + and HCI has been investigated by Glenewinkel-Meyer and Ottinger [34].

After Ar+HC1 is formed, excess energy of the system may be released by luminescence. However, charge transfer and vibrational predissociation in clusters should be faster than the above process. Therefore, the charge transfer and vibrational predis- sociation processes should take place in the system, and these are considered to be the main cause that the photoionization cross-section of ArHC1 is appar- ently lower than that expected in the wavelength region where Ar begins to be ionized.

We expect that the above photoionization-charge transfer-dissociation processes should take place easily in van der Waals clusters when the difference between the subunits' ionization potential is rela- tively large. By this method, one can investigate charge transfer processes and prepare some special excited states of molecules.

4. Conclusion

The ArHC1 cluster has been investigated using a synchrotron radiation light source and PIMS. The results show that the concentration of ArHC1 in a supersonic expansion is approximately given by c~(ArHC1)% = 1.79 × 10 .8 P015, where P0 is the nozzle stagnation pressure in Pa.

The PIE curve of ArHC1 has been obtained. The appearance potential of ArHCI + was determined to be 12.52 _+ 0.03 eV and the bond dissociation energy D O of ArHC1 + is deduced to be 0.24 _+ 0.04 eV. The theoretical calculations show that both ionic and neutral ArHC1 have a linear conformation and after the cluster is ionized, the dissociation energy has a large increase.

The photoionization mechanism of ArHC1 is dis- cussed. When Ar in ArHC1 is ionized, the charge transfer and vibrational predissociation processes are proposed.

Acknowledgements

This work was supported by the Natural Science Foundation of China.

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