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Two-photon spectroscopy of autoionizing states of Xe2 near threshold
Volume 165, number 2,3
TWO-PHOTON
CHEMICAL PHYSICS LETTERS
SPECTROSCOPY OF AUTOIONIZING
12 January 1990
STATES OF Xez NEAR THRESHOLD *
ST. PRATT, P.M. DEHMER and J.L. DEHMER Argonne National Laboratory, Argonne, IL 60439, USA Received I7 October 1989
The two-photon ionization spectrum of Xez in the region of the first ionization threshold is presented. Vibronic bands corresponding to at least four different autoionizing electronic states of Xe, are observed for the first time and are tentatively assigned. The observed appearance potential is significantly higher (by 415 cm-‘) than the earlier single-photon ionization result (C.Y. Ng,D.J. Trevor, B.H. Mahan andY.T. Lee, J. Chem. Phys. 65 (1976) 4327).
1. Introduction Single-photon ionization studies of the rare gas van der Waals molecules have revealed a wealth of information on the ionization potentials of the neutral dimers and on the dissociation energies of both excited and ionic neutral states [ l-3 1. Detailed studies of the dissociation energies as a function of bonding partners have revealed systematic trends for these weakly bound species [ l-31, and new theoretical techniques have been developed in an attempt to understand these systematic trends in the heteronuclear rare gas dimers [ 41. The first photoionization mass spectromettic study of a rare gas van der Waals molecule was performed on XeZ by Ng et al. [ 51 in 1976. They examined the region from the first ionization potential to just above the atomic Xe+ *PO 3,2 threshold. Because the resolution employed in that study was relatively low, their spectrum in this region could provide little evidence for resonant autoionizing features. Such features are expected and have been observed in the threshold regions of the spectra of the other homonuclear dimers Ne, [6], Ar, [7-91, and Kr2 [7,10]. AlthoughNorwood et al. [ 111 have recently performed a photoelectron-photoion coincidence study of the Xe, ionization threshold, the resolution was comparable to * Work supported by the US Department of Energy, Offtce of Health and Environmental Research and Office of Arms Control, under Contract W-3 1- I09-ENG-38. 0009-2614/90/$
the earlier work [5]. To date, no high-resolution, single-photon or multiphoton study of Xe2 near the ionization threshold has been reported. In the present paper we report the two-photon ionization spectrum of XeZ near the first ionization threshold. In this ionization process the first photon is nonresonant, and all of the observed structure corresponds to features at the two-photon energy, which was scanned from several hundred cm-’ below the ionization threshold to several hundred cm-’ above it. Because a laser excitation source is used, the wavelength resolution is quite high. As a result, far more of the resonant autoionization structure is observed than was observed in the low-resolution, single-photon ionization study [ 5 1. However, because of the gerade/ungerade symmetry present in homonuclear molecules, the autoionizing levels accessed by two-photon excitation of Xe2 are different from those accessed by single-photon ionization [ 121. Earlier two-photon laser-induced fluorescence and two-photon resonant, three-photon ionization (2 + 1) studies of Xez [ 13- 181 and the collisional quenching of Xe [ 19-241 have been performed at lower energies in the region of the atomic Xe 6p and 5d levels, and the present work extends these studies to above the Xez ionization threshold. Bands corresponding to at least four new autoionizing states are observed above the ionization threshold, and partial analyses of these bands are presented. In addition, we discuss the observed appearance potential of Xez in this two-photon ionization experiment and
03.50 0 Elsevler Science Publishers B.V. (North-Holland)
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CHEMICAL
12January1990
PHYSICSLETTERS
compare it to the appearance potential observed in the single-photon ionization experiment of Ng et al.
3. Results and discussion
151.
The threshold region of the two-photon ionization spectrum of Xe, is shown in fig. 1. The spectrum displays a clear onset for ionization at 90125 cm-‘. A number of autoionizing features are also observed, and some of these fall into regular vibrational progressions. The two-photon atomic transitions to the 4f[ 3/2], and 4f[5/2], levels are indicated in the figure [28]. The discussion of the spectrum is divided into two parts. The electric field dependence of the ionization threshold and a discussion of the appearance potential of Xe: are presented in section 3.1, and a partial analysis of the new autoionizing structure is presented in section 3.2.
2. Experiment Spectra were obtained by using two different instruments The first combines a time-of-flight mass spectrometer and hemispherical electron kinetic energy analyzer and has been described previously [ 251. The second is a differentially pumped time-offlight mass spectrometer similar to that of Opsal et al. [ 261; a detailed description of this instrument will be published later. A constant ion drawout voltage of = 20- 120 V/cm was used in both instruments. In addition, both instruments used an unskimmed 35 l,trn diameter supersonic molecular beam produced by expanding z 1 atm of pure Xe. These expansion conditions produce a significant concentration of Xez with negligible amounts of heavier clusters [ 16,17,27]. Ultraviolet light from 225.0 to 218.5 nm was generated in P-BazBz04 by frequency-doubling or -tripling the output of an Nd: YAG-pumped dye laser. The resulting ultraviolet light (50-100 l.tJ) was separated from the other beams by using a series of four 60” prisms arranged to maintain a constant beam direction and was focused into the interaction region by using a 150 mm focal length, fused silica lens. The spectra of Xe2 were recorded by scanning the wavelength of the light while monitoring the Xe: ion signal. The wavelength calibration was performed by using two-photon transitions in atomic Xe [28] that occur throughout the wavelength region of the study. (Two-photon transitions to very high Rydberg states of atomic Xe have recently been reported [29].) These transitions lead to intense (2 + 1) ionization and are observed in the Xe: mass peak as a result of tailing of the Xe+ ion signal into the Xe: mass peak. Thus, the atomic transitions are observed along with the dimer transitions. The use of an unskimmed jet introduces the possibility of collisions, and chemiionization of excited Xe atoms also produces Xe: and contributes to the observation of atomic transitions in the Xe: spectrum.
132
3.1. The ionization potential of Xe2 The determination of accurate ionization potentials from appearance potentials is not straightforward. In general, the effects of collisions [ 301, finite resolution [ 3 1,321. thermal population distributions [ 31,321, and external fields [ 331 must be considered. Although many of these difftculties have been overcome in careful zero-kinetic-energy photoelectron spectra of NO [ 341, when possible, the ex-
v’:
m+2
ill
I
Yh 0
’
2 ‘l
--JJ(glJ. I
90000
I
I
I
I
I
90500
I
91000
TWO-PHOTON ENERGY (cm-‘) Fig. 1. The two-photon ionization spectrum of Xe, obtained by monitoring the Xe,’ ion signal in the vicinity of the first ionization threshold. The spectrum is offsetfrom the x’axis for clarity; the signal level from 90000 to 90100 cm-’ corresponds to the background level. The atomic 4f transitions are observed by chemiionization and by tailing of the much more intense Xe+ mass peak into the Xe: channel.
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CHEMICALPHYSICSLETTERS
trapolation of Rydberg series converging to the ground state of the ion is the preferred technique [ 121. Unfortunately, no such series in Xe, have been observed to high principal quantum number. Thus, in the present work and in the earlier work of Ng et al. [ 51, the determination of the ionization potential relies on the measurement of the onset of ionization under nonideal conditions. Our earlier studies of Xez using similar molecular beam conditions showed that hot-band transitions from vibrationally excited levels of the ground electronic state are significantly weaker than the v” =O transitions [ 16,171; thus, the sharp onset in the present experiment is most likely due to excitation of vibrationally cold molecules. The finite band width of the laser is very much smaller than the accuracy of the present determination and is therefore ignored. The effect of collisional ionization on the measured ionization threshold is more difficult to assess, as the predissociation and fluorescence lifetimes of the high Rydberg states of Xe; are unknown. If both of these lifetimes are long compared to the mean collision time of Xez molecules in the molecular beam, the ionization threshold would be expected to be decreased by an amount on the order of the average kinetic energy of the molecules in the beam [ 30,35 1, i.e. approximately 200 cm-‘. As will be discussed below, this does not appear to be the case, indicating that the fluorescence or, more likely, the predissociation lifetime of the high Rydberg states of Xe; is much shorter than the mean collision time. The dc drawout field used in this experiment does have a large effect on the observed ionization threshold. Classically, the decrease in the ionization threshold due to an external electric field is proportional to the square root of the field, F [ 331. Threshold shifts, d, are usually fitted to the equation [ 31% 411 A=cxF”~,
(1)
where A and F are typically given in cm-’ and V/ cm, respectively. The Xer appearance potential was determined as a function f drawout field for values of F from 25 to 1 IS V/cm. A fit of the data to eq. (1) gives a value of cr=-7.4kO.5 cm-‘/(V/ cm)‘/2. This is larger than the classical value of a=-6.11 cm-‘/(V/cm)“2 for a pure Coulomb field [ 331 and is also larger than values for a number
12January 1990
of other molecules [ 36-411. Using this value of cr, the appearance potential was extrapolated back to F=O, yielding a zero field appearance potential of 90157+ 10 cm-‘. This present value for the appearance potential is significantly higher than the value [ 5 ] of 89742 + 80 cm- ’ reported by Ng et al. [ 51. Although the oneand two-photon ionization spectra are not directly comparable because they access opposite parity continua, the ionization potentials determined by the two techniques should be equal. Ng et al. [ 51 did not report the value of the electric field in the ionization region in their earlier study, but it is unlikely to be sufficient to account for the observed 415 cm- ’ difference. The experiments of Ng et al. [ 51 were performed by using a skimmed supersonic molecular beam source, so collisional effects were not important. There is no obvious reason why the determination of the ionization potential by Ng et al. [ 51 would yield a value that was smaller than the actual value, and we are not in a position to analyze the possible errors in their experiment. The simplest explanation for the present result is that the two-photon ionization cross section of Xe, is too small to observe significant ionization until the observed onset, well above the adiabatic ionization potential. This could be due to a combination of small electronic transition matrix elements and small Franck-Condon factors at threshold. The theoretical [ 42,431 u, value of 123 cm- ’ for the Xe$ A ‘2: ground state implies that the onset in the two-photon ionization spectrum of fig. 1. occurs at the v+ = 3 or possibly the t’+ =4 threshold if one assumes that the one-photon threshold is correct. It is also possible that autoionizing, ungerade Rydberg states enhance the ionization cross section at threshold in the spectrum of Ng et al. [ 51. It is interesting that the spectrum of Ng et al. [ 51 displays only a very weak rise at the quoted threshold of 111.43 + 0.1 nm, and that a much stronger rise isobservedat ~110.95+0.lnm (90131+80cm-‘), in much better accord with the onset observed in the present experiment. 3.2. Moleculur band systems near threshold The ground and excited electronic states of the rare gas dimers are most often described in Hund’s case 133
Volume 165,number 2,3
(c) [ 121, in which the good quantum number is 9, the projection of the total angular momentum of the electrons onto the internuclear axis. The rotational angular momentum of the nuclei N is coupled to 52 to give the total angular momentum J, however, rotational structure is not resolved in any of the observed bands, so the following discussion is confined to vibronic symmetry. The ground electronic state of Xez has 0: symmetry, and the allowed two-photon transitions are to states with 0:) l,, and 2, symmetry ( 1Al21 6 2 ). In general, the strongest molecular transitions are expected [ 16-l 81 to be to states that are derived from a ground state Xe atom and an excited Xe atom in a two-photon-allowed state. As is seen in fig. 1, two two-photon-allowed atomic transitions occur in the wavelength region of the present study [ 281, corresponding to the ( 2P$2)4f[ 3/2], and (2P&,)4f[ 5/2], states. An atom in either of these states can be coupled with a ground state atom to produce three two-photon-allowed molecular states, corresponding to 0:) l,, and 2,. Thus, there is a total of six possible electronic states with these two limits. Six two-photon forbidden (‘P&,)4f atomic levels also occur in this region (281; in addition, the forbidden (2Pz,2)6d[3/2], ,
(2P;lz)Ss[3/2],, and (‘P&)8s[3/2], levels lie at 90032.65, 90805.045, and 90932.939 cm-‘, respectively [ 281. Molecular states with 0:) l,, or 2, symmetry can be derived from all of these levels, and although these states are expected to have smaller oscillator strengths than those derived from allowed limits, the interactions between these states and those produced from allowed limits may produce considerable perturbations in the spectrum. The progression given in table 1 consists of five bands beginning at 90235.0 cm-‘. The first band has considerable intensity and is assigned as the (v’ =O, v” = 0) transition. The band spacing is quite irregular with an w, value of z 37 cm-‘. No conclusive evidence is observed for bands with v” > 0. If the dissociation limit is known, the dissociation energy, Do, of the excited electronic state can be calculated from the (0, 0) band energy by using &(XeT)=E(Xe*)+&(Xe,)-E(Xe;),
(2)
where E( Xe*) is the energy of the atomic Xe* level 134
12January 1990
CHEMICALPHYSICSLETTERS Table 1 Progression 1 V’
Two-photon
AG:+,,z
energy(cm-‘) 0 0 0 0 0
0 I 2 3 4
90235.0 90270.2 90302.4 90324.0 90350.6
35.2 32.2 21.6 26.6
0,=800.1 cm-’ for dissociationto Xe IS,+ Xe*(‘I$)4f[3/2]2 0,=860.7cm-‘fordissociationtoXe’%tXe*(’~,,)4f[5/2]*
Table 2 Progression2
Vf’
V’
Two-photon energy (cm-‘)
AG+,,,
0 0 0 0 0
m mtl mt2 mt3 m+4
90552.3 90625.0 90691.5 90749.4 90809.2
72.7 66.5 51.9 59.8
D,=482.8 cm-’ for dissociation to Xe’S,,tXe*(ZPj,2)4f[3/2]z D,= 543.4cm-’ for dissociationto Xe IS,t Xe* (*P&)4f[ 5121,
(the dissociation
limit), Do(Xe2) is the dissociation
energy of the 0: ground state (here taken as 185.16 cm-‘) [44],andE(XeS)istheenergyofthe(O,O) transition. If the state dissociates to XeS Xe*(‘PQ2)4f[3/2], orXe(2P$2)4f[5/2]2,thedissociation energy is 800.1 or 860.7 cm-‘, respectively. The progression given in table 2 consists of five bands beginning with a weak transition at 90552.3 cm-‘. The envelope of the progression suggests that the lowest energy band observed may not correspond to the (0, 0) transition, and we have labelled the first band (0, m). Again, there is no evidence for hot bands. If the state dissociates to Xe+ Xe*(*P’&,)4f[ 3/2], or Xe*(‘P$)4f[ 5/2],, the dissociation energy from the v’ = m level is 482.8 or 543.4 cm-‘, respectively. The intense band appearing at 90365.8 cm-’ does not fit into the progression of table 1. This band is most likely a (0,O) transition as it appears to be unrelated to neighboring bands. The calculated disso-
Volume 165, number 2,3
CHEMICALPHYSICSLETTERS
ciation energies are 669.3 and 729.9 cm-’ for the Xe+Xe*(2P&)4f[3/2]2 and (2P$2)4f[5/2]2 limits, respectively. A second very intense band that appears to be unrelated to neighboring bands is observed at 90610.6 cm-‘. Assuming that this is also a (0, 0) band with the upper state dissociating to Xe+Xe*(2P812)4f[3/2], or (‘P&)4f[5/212 gives dissociation energies of 424.5 and 485.1 cm-‘, respectively. Several very weak bands that occur between 90400 and 90600 cm-’ will not be discussed. The assignment of 9 values to each of the four electronic states observed cannot be made with the information available at this time. Although a propensity rule that transitions with A.&O are stronger than those with IA.Ql> 0 has been discussed [ 16,17,43,45 1, Lipson et al. [ 18 ] have recently provided clear evidence to the contrary for transitions to Rydberg states of Xe,. All of the dissociation energies calculated here are between 400 and 900 cm-‘, indicating that the ion cores are most likely either the Xe$ B211,,Zg or C211J,2U electronic states [ 17,42,46 1. If the excited electron for these states of Xe2 has predominantly 4f character, the ion core must have predominantly C 217j,zUcharacter, as the Xe, excited state must have g symmetry. However, as was discussed above, there are a number of 6d and 8s states of atomic Xe in this energy region. States of Xe, with g symmetry and even 1Rydberg electrons must have ion cores with g symmetry. Configuration interaction will mix (C211,,,,)nf states with (B 211,,2.&s and nd states, and the dissociation energies will be affected accordingly. The irregularity of the vibrational progression gives some evidence for such interactions.
4. Conclusions The two-photon ionization spectrum of Xe2 in the region of the first ionization threshold displays a sharp onset for direct ionization as well as autoionizing resonances corresponding to at least four different electronic states. The onset for direct ionization is significantly higher than in the single-photon ionization spectrum recorded by Ng et al. [ 5 1. Assuming that the earlier measurement is correct, we must conclude that the two-photon ionization cross section at the adiabatic ionization potential is small,
12January 1990
although the distinct step in the two-photon spectrum would indicate that the cross section does not decrease smoothly towards the adiabatic threshold. Analysis of the autoionizing resonances indicates that the observed electronic states most likely have Xe: C 2113i2Uion cores, although configuration interaction with states having B 21’13,29 ion cores may be responsible for irregularities in the spectrum. The effects of such configuration interaction may become far more pronounced if the spectrum can be extended to the B 2113,28and C21JJ,2,, ionization thresholds. Such studies will require higher laser power because of the low intensities of these series at high principal quantum number. Higher power studies should also allow the observation and a more accurate determination of the adiabatic ionization potential.
References [ 1] C.Y. Ng, Advan. Chem. Phys. 52 (1983) 263. [2] P.M. Dehmer and S.T. Pratt, in: Photophysics and photochemistry in the vacuum ultraviolet, eds. S.P. McGlynn, G.L. Findlay and R.H. Huebner (Reidel, Dordrecht, 1985) p. 467. [ 31 P.M. Dehmerand S.T. Pratt, J. Chem. Phys. 77 (1982) 4804. [4] N.Y. Du and C.H. Greene, J. Chem. Phys. 90 (1989) 6347. [5] C.Y. Ng, D.J. Trevor, B.H. Mahan and Y.T. Lee, J. Chem. Phys. 65 (1976) 4327. [6] D.J. Trevor, I.E. Pollard, W.D. Brewer, S.H. Southworth, C.M. Truesdale, D.A. Shirley and Y.T. Lee, J. Chem. Phys. 80 (1984) 6083. [71 C.Y. Ng, D.J. Trevor, B.H. Mahan and Y.T. Lee, .I. Chem. Phys. 66 (1977) 446. [B] P.M. Dehmer and E.D. Poliakoff, Chem. Phys. Letters 77 (1981) 326. [9] P.M. Dehmer, J. Chem. Phys. 76 (1982) 1263. [ lo] XT. Pratt and P.M. Dehmer, Chem. Phys. Letters 87 (1982) 533. [ II] K. Norwood, G. Luo and C.Y. Ng, J. Chem. Phys. 90 ( 1989) 4689. [ 121 G. He&erg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [ 131 W. Gomik, S. Kind& E. Matthias and D. Schmidt, J. Chem. Phys. 75 ( 198 1) 68. [ 141 W. Gomik, E. Matthias and D. Schmidt, J. Phys. B I5 (1982) 3413. [ 15 ] T.D. Raymond, N. Bower& C.Y. Kuo and J.W. Keto, Phys. Rev. A29 (1984) 721. [ 161 P.M. Dehmer, ST. Pratt and J.L. Dehmer, J. Chem. Phys. 85 (1986) 13.
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CHEMICAL PHYSICS LETTERS
[ 171 P.M. Dehmer, ST. Pratt and J.L. Dehmer, J. Phys. Chem. 91 (1987) 2593. [ 181 R.H. Lipson, A.R. Hoy and E. Chart, J. Chem. Phys. 90 ( 1989) 4664. [ 191 N. Bowering T.D. Raymond and J.W. Keto, Phys. Rev. Letters 52 (1984) 1880. [ 201 N. Bowering, M.R. Bruce and J.W. Keto, J. Chem. Phys. 84 ( 1986) 709. [ 2 1 ] N. Bower@ M.R. Bruce and J.W. Keto, J. Chem. Phys. 84 (1986) 715. [ 221 J.K. Ku and D.W. Setser, J. Chem. Phys. 84 (1986) 4304. [23] P. Moutard, P. Laporte, N. Damany, J.L. Subtil and H. Damany,Chem. Phys. Letters 132 (1986) 521. [24] P. Moutard, P. Laporte, J.L. Subtil, N. Damany and M. Damany, J. Chem. Phys. 88 ( 1988) 7485. [25] ST. Pratt, E.D. Poliakoff, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 78 (1983) 65. [ 261 R.B. Opsal, KG. Owens and J.P. Reilly, .4nal. Chem. 57 (1985) 1884. [27] P.M. Dehmer and S.T. Pratt, J. Chem. Phys. 75 (1981) 5265. [ 281 C.E. Moore, Atomic energy levels, Vol. 3, NBS Circular No. 467 (US GPO, Washington, 1949). [ 291 A. L’Huillier, L.A. Lompre, D. Normand, J. Morellee, M. Ferray, J. Lavancier, G. Mainfray and C. Manus, J. Opt. Sot. Am. B 6 (1989) 1644. [ 301 R.E. Stebbings and F.B. Dunning, eds., Rydberg states of atoms and molecules (Cambridge Univ. Press, Cambridge, 1983).
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[ 311 W.A. Chupka, J. Chem. Phys. 54 (1971) 1936.
[ 321 P.M.Guyon and J. Berkowitz, J. Chem. Phys. 54 ( 1971) 1814.
[ 331 H. Metcalf, Nature 284 (1980) 127. [ 341 G. Reiser, W. Habenicht, K. Miiller-Dethlefs and E.W. Schlag, Chem. Phys. Letters 152 (1988) 119.
[ 351 K. Radler and J. Berkowitz, I. Chem. Phys. 70 (1979) 221. [ 361 M.A. Duncan, T.G. Dietz and R.E. Smalley, J. Chem. Phys. 75 (1981) 2118.
[ 371 K.H. Fung, H.L. Selzle and E.W. Schlag, Z. Naturforsch. A 36 (1981) 1257. [38] E.D. Poliakoff, J.L. Dehmer, A.C. Parr and GE. Leroi, Chem. Phys. Letters 111 (1984) 128. [39] Ch. Bordas, P. Brevet, M. Broyer, J. Chevaleyre and P. Labastie, Europhys. Letters 3 ( 1987) 789. [40] T. Ebata, Y. Anezaki, M. Fujii, N. Mikami and M. Ito, J. Phys. Chem. 87 (1983) 4773. [41] S.T. Pratt, J.L. Dehmer and P.M. Dehmer, J. Chem. Phys. 90 (1989) 2201. [42] P.M. Dehmer and J.L. Dehmer, I. Chem. Phys. 68 (1978) 3462. [43] W.R. Wadt, J. Chem. Phys. 68 (1978) 402. [44] D.E. Freeman, K. Yoshino and Y. Tanaka, J. Chem. Phys. 61 (1974) 4880. [45] P.J. Hay and T.H. Dunning Jr., I. Chem. Phys. 66 (1977) 1306. [46 j R.S. Mulliken, J. Chem. Phys. 52 (1970) 5 170.
TWO-PHOTON
CHEMICAL PHYSICS LETTERS
SPECTROSCOPY OF AUTOIONIZING
12 January 1990
STATES OF Xez NEAR THRESHOLD *
ST. PRATT, P.M. DEHMER and J.L. DEHMER Argonne National Laboratory, Argonne, IL 60439, USA Received I7 October 1989
The two-photon ionization spectrum of Xez in the region of the first ionization threshold is presented. Vibronic bands corresponding to at least four different autoionizing electronic states of Xe, are observed for the first time and are tentatively assigned. The observed appearance potential is significantly higher (by 415 cm-‘) than the earlier single-photon ionization result (C.Y. Ng,D.J. Trevor, B.H. Mahan andY.T. Lee, J. Chem. Phys. 65 (1976) 4327).
1. Introduction Single-photon ionization studies of the rare gas van der Waals molecules have revealed a wealth of information on the ionization potentials of the neutral dimers and on the dissociation energies of both excited and ionic neutral states [ l-3 1. Detailed studies of the dissociation energies as a function of bonding partners have revealed systematic trends for these weakly bound species [ l-31, and new theoretical techniques have been developed in an attempt to understand these systematic trends in the heteronuclear rare gas dimers [ 41. The first photoionization mass spectromettic study of a rare gas van der Waals molecule was performed on XeZ by Ng et al. [ 51 in 1976. They examined the region from the first ionization potential to just above the atomic Xe+ *PO 3,2 threshold. Because the resolution employed in that study was relatively low, their spectrum in this region could provide little evidence for resonant autoionizing features. Such features are expected and have been observed in the threshold regions of the spectra of the other homonuclear dimers Ne, [6], Ar, [7-91, and Kr2 [7,10]. AlthoughNorwood et al. [ 111 have recently performed a photoelectron-photoion coincidence study of the Xe, ionization threshold, the resolution was comparable to * Work supported by the US Department of Energy, Offtce of Health and Environmental Research and Office of Arms Control, under Contract W-3 1- I09-ENG-38. 0009-2614/90/$
the earlier work [5]. To date, no high-resolution, single-photon or multiphoton study of Xe2 near the ionization threshold has been reported. In the present paper we report the two-photon ionization spectrum of XeZ near the first ionization threshold. In this ionization process the first photon is nonresonant, and all of the observed structure corresponds to features at the two-photon energy, which was scanned from several hundred cm-’ below the ionization threshold to several hundred cm-’ above it. Because a laser excitation source is used, the wavelength resolution is quite high. As a result, far more of the resonant autoionization structure is observed than was observed in the low-resolution, single-photon ionization study [ 5 1. However, because of the gerade/ungerade symmetry present in homonuclear molecules, the autoionizing levels accessed by two-photon excitation of Xe2 are different from those accessed by single-photon ionization [ 121. Earlier two-photon laser-induced fluorescence and two-photon resonant, three-photon ionization (2 + 1) studies of Xez [ 13- 181 and the collisional quenching of Xe [ 19-241 have been performed at lower energies in the region of the atomic Xe 6p and 5d levels, and the present work extends these studies to above the Xez ionization threshold. Bands corresponding to at least four new autoionizing states are observed above the ionization threshold, and partial analyses of these bands are presented. In addition, we discuss the observed appearance potential of Xez in this two-photon ionization experiment and
03.50 0 Elsevler Science Publishers B.V. (North-Holland)
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PHYSICSLETTERS
compare it to the appearance potential observed in the single-photon ionization experiment of Ng et al.
3. Results and discussion
151.
The threshold region of the two-photon ionization spectrum of Xe, is shown in fig. 1. The spectrum displays a clear onset for ionization at 90125 cm-‘. A number of autoionizing features are also observed, and some of these fall into regular vibrational progressions. The two-photon atomic transitions to the 4f[ 3/2], and 4f[5/2], levels are indicated in the figure [28]. The discussion of the spectrum is divided into two parts. The electric field dependence of the ionization threshold and a discussion of the appearance potential of Xe: are presented in section 3.1, and a partial analysis of the new autoionizing structure is presented in section 3.2.
2. Experiment Spectra were obtained by using two different instruments The first combines a time-of-flight mass spectrometer and hemispherical electron kinetic energy analyzer and has been described previously [ 251. The second is a differentially pumped time-offlight mass spectrometer similar to that of Opsal et al. [ 261; a detailed description of this instrument will be published later. A constant ion drawout voltage of = 20- 120 V/cm was used in both instruments. In addition, both instruments used an unskimmed 35 l,trn diameter supersonic molecular beam produced by expanding z 1 atm of pure Xe. These expansion conditions produce a significant concentration of Xez with negligible amounts of heavier clusters [ 16,17,27]. Ultraviolet light from 225.0 to 218.5 nm was generated in P-BazBz04 by frequency-doubling or -tripling the output of an Nd: YAG-pumped dye laser. The resulting ultraviolet light (50-100 l.tJ) was separated from the other beams by using a series of four 60” prisms arranged to maintain a constant beam direction and was focused into the interaction region by using a 150 mm focal length, fused silica lens. The spectra of Xe2 were recorded by scanning the wavelength of the light while monitoring the Xe: ion signal. The wavelength calibration was performed by using two-photon transitions in atomic Xe [28] that occur throughout the wavelength region of the study. (Two-photon transitions to very high Rydberg states of atomic Xe have recently been reported [29].) These transitions lead to intense (2 + 1) ionization and are observed in the Xe: mass peak as a result of tailing of the Xe+ ion signal into the Xe: mass peak. Thus, the atomic transitions are observed along with the dimer transitions. The use of an unskimmed jet introduces the possibility of collisions, and chemiionization of excited Xe atoms also produces Xe: and contributes to the observation of atomic transitions in the Xe: spectrum.
132
3.1. The ionization potential of Xe2 The determination of accurate ionization potentials from appearance potentials is not straightforward. In general, the effects of collisions [ 301, finite resolution [ 3 1,321. thermal population distributions [ 31,321, and external fields [ 331 must be considered. Although many of these difftculties have been overcome in careful zero-kinetic-energy photoelectron spectra of NO [ 341, when possible, the ex-
v’:
m+2
ill
I
Yh 0
’
2 ‘l
--JJ(glJ. I
90000
I
I
I
I
I
90500
I
91000
TWO-PHOTON ENERGY (cm-‘) Fig. 1. The two-photon ionization spectrum of Xe, obtained by monitoring the Xe,’ ion signal in the vicinity of the first ionization threshold. The spectrum is offsetfrom the x’axis for clarity; the signal level from 90000 to 90100 cm-’ corresponds to the background level. The atomic 4f transitions are observed by chemiionization and by tailing of the much more intense Xe+ mass peak into the Xe: channel.
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trapolation of Rydberg series converging to the ground state of the ion is the preferred technique [ 121. Unfortunately, no such series in Xe, have been observed to high principal quantum number. Thus, in the present work and in the earlier work of Ng et al. [ 51, the determination of the ionization potential relies on the measurement of the onset of ionization under nonideal conditions. Our earlier studies of Xez using similar molecular beam conditions showed that hot-band transitions from vibrationally excited levels of the ground electronic state are significantly weaker than the v” =O transitions [ 16,171; thus, the sharp onset in the present experiment is most likely due to excitation of vibrationally cold molecules. The finite band width of the laser is very much smaller than the accuracy of the present determination and is therefore ignored. The effect of collisional ionization on the measured ionization threshold is more difficult to assess, as the predissociation and fluorescence lifetimes of the high Rydberg states of Xe; are unknown. If both of these lifetimes are long compared to the mean collision time of Xez molecules in the molecular beam, the ionization threshold would be expected to be decreased by an amount on the order of the average kinetic energy of the molecules in the beam [ 30,35 1, i.e. approximately 200 cm-‘. As will be discussed below, this does not appear to be the case, indicating that the fluorescence or, more likely, the predissociation lifetime of the high Rydberg states of Xe; is much shorter than the mean collision time. The dc drawout field used in this experiment does have a large effect on the observed ionization threshold. Classically, the decrease in the ionization threshold due to an external electric field is proportional to the square root of the field, F [ 331. Threshold shifts, d, are usually fitted to the equation [ 31% 411 A=cxF”~,
(1)
where A and F are typically given in cm-’ and V/ cm, respectively. The Xer appearance potential was determined as a function f drawout field for values of F from 25 to 1 IS V/cm. A fit of the data to eq. (1) gives a value of cr=-7.4kO.5 cm-‘/(V/ cm)‘/2. This is larger than the classical value of a=-6.11 cm-‘/(V/cm)“2 for a pure Coulomb field [ 331 and is also larger than values for a number
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of other molecules [ 36-411. Using this value of cr, the appearance potential was extrapolated back to F=O, yielding a zero field appearance potential of 90157+ 10 cm-‘. This present value for the appearance potential is significantly higher than the value [ 5 ] of 89742 + 80 cm- ’ reported by Ng et al. [ 51. Although the oneand two-photon ionization spectra are not directly comparable because they access opposite parity continua, the ionization potentials determined by the two techniques should be equal. Ng et al. [ 51 did not report the value of the electric field in the ionization region in their earlier study, but it is unlikely to be sufficient to account for the observed 415 cm- ’ difference. The experiments of Ng et al. [ 51 were performed by using a skimmed supersonic molecular beam source, so collisional effects were not important. There is no obvious reason why the determination of the ionization potential by Ng et al. [ 51 would yield a value that was smaller than the actual value, and we are not in a position to analyze the possible errors in their experiment. The simplest explanation for the present result is that the two-photon ionization cross section of Xe, is too small to observe significant ionization until the observed onset, well above the adiabatic ionization potential. This could be due to a combination of small electronic transition matrix elements and small Franck-Condon factors at threshold. The theoretical [ 42,431 u, value of 123 cm- ’ for the Xe$ A ‘2: ground state implies that the onset in the two-photon ionization spectrum of fig. 1. occurs at the v+ = 3 or possibly the t’+ =4 threshold if one assumes that the one-photon threshold is correct. It is also possible that autoionizing, ungerade Rydberg states enhance the ionization cross section at threshold in the spectrum of Ng et al. [ 51. It is interesting that the spectrum of Ng et al. [ 51 displays only a very weak rise at the quoted threshold of 111.43 + 0.1 nm, and that a much stronger rise isobservedat ~110.95+0.lnm (90131+80cm-‘), in much better accord with the onset observed in the present experiment. 3.2. Moleculur band systems near threshold The ground and excited electronic states of the rare gas dimers are most often described in Hund’s case 133
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(c) [ 121, in which the good quantum number is 9, the projection of the total angular momentum of the electrons onto the internuclear axis. The rotational angular momentum of the nuclei N is coupled to 52 to give the total angular momentum J, however, rotational structure is not resolved in any of the observed bands, so the following discussion is confined to vibronic symmetry. The ground electronic state of Xez has 0: symmetry, and the allowed two-photon transitions are to states with 0:) l,, and 2, symmetry ( 1Al21 6 2 ). In general, the strongest molecular transitions are expected [ 16-l 81 to be to states that are derived from a ground state Xe atom and an excited Xe atom in a two-photon-allowed state. As is seen in fig. 1, two two-photon-allowed atomic transitions occur in the wavelength region of the present study [ 281, corresponding to the ( 2P$2)4f[ 3/2], and (2P&,)4f[ 5/2], states. An atom in either of these states can be coupled with a ground state atom to produce three two-photon-allowed molecular states, corresponding to 0:) l,, and 2,. Thus, there is a total of six possible electronic states with these two limits. Six two-photon forbidden (‘P&,)4f atomic levels also occur in this region (281; in addition, the forbidden (2Pz,2)6d[3/2], ,
(2P;lz)Ss[3/2],, and (‘P&)8s[3/2], levels lie at 90032.65, 90805.045, and 90932.939 cm-‘, respectively [ 281. Molecular states with 0:) l,, or 2, symmetry can be derived from all of these levels, and although these states are expected to have smaller oscillator strengths than those derived from allowed limits, the interactions between these states and those produced from allowed limits may produce considerable perturbations in the spectrum. The progression given in table 1 consists of five bands beginning at 90235.0 cm-‘. The first band has considerable intensity and is assigned as the (v’ =O, v” = 0) transition. The band spacing is quite irregular with an w, value of z 37 cm-‘. No conclusive evidence is observed for bands with v” > 0. If the dissociation limit is known, the dissociation energy, Do, of the excited electronic state can be calculated from the (0, 0) band energy by using &(XeT)=E(Xe*)+&(Xe,)-E(Xe;),
(2)
where E( Xe*) is the energy of the atomic Xe* level 134
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CHEMICALPHYSICSLETTERS Table 1 Progression 1 V’
Two-photon
AG:+,,z
energy(cm-‘) 0 0 0 0 0
0 I 2 3 4
90235.0 90270.2 90302.4 90324.0 90350.6
35.2 32.2 21.6 26.6
0,=800.1 cm-’ for dissociationto Xe IS,+ Xe*(‘I$)4f[3/2]2 0,=860.7cm-‘fordissociationtoXe’%tXe*(’~,,)4f[5/2]*
Table 2 Progression2
Vf’
V’
Two-photon energy (cm-‘)
AG+,,,
0 0 0 0 0
m mtl mt2 mt3 m+4
90552.3 90625.0 90691.5 90749.4 90809.2
72.7 66.5 51.9 59.8
D,=482.8 cm-’ for dissociation to Xe’S,,tXe*(ZPj,2)4f[3/2]z D,= 543.4cm-’ for dissociationto Xe IS,t Xe* (*P&)4f[ 5121,
(the dissociation
limit), Do(Xe2) is the dissociation
energy of the 0: ground state (here taken as 185.16 cm-‘) [44],andE(XeS)istheenergyofthe(O,O) transition. If the state dissociates to XeS Xe*(‘PQ2)4f[3/2], orXe(2P$2)4f[5/2]2,thedissociation energy is 800.1 or 860.7 cm-‘, respectively. The progression given in table 2 consists of five bands beginning with a weak transition at 90552.3 cm-‘. The envelope of the progression suggests that the lowest energy band observed may not correspond to the (0, 0) transition, and we have labelled the first band (0, m). Again, there is no evidence for hot bands. If the state dissociates to Xe+ Xe*(*P’&,)4f[ 3/2], or Xe*(‘P$)4f[ 5/2],, the dissociation energy from the v’ = m level is 482.8 or 543.4 cm-‘, respectively. The intense band appearing at 90365.8 cm-’ does not fit into the progression of table 1. This band is most likely a (0,O) transition as it appears to be unrelated to neighboring bands. The calculated disso-
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ciation energies are 669.3 and 729.9 cm-’ for the Xe+Xe*(2P&)4f[3/2]2 and (2P$2)4f[5/2]2 limits, respectively. A second very intense band that appears to be unrelated to neighboring bands is observed at 90610.6 cm-‘. Assuming that this is also a (0, 0) band with the upper state dissociating to Xe+Xe*(2P812)4f[3/2], or (‘P&)4f[5/212 gives dissociation energies of 424.5 and 485.1 cm-‘, respectively. Several very weak bands that occur between 90400 and 90600 cm-’ will not be discussed. The assignment of 9 values to each of the four electronic states observed cannot be made with the information available at this time. Although a propensity rule that transitions with A.&O are stronger than those with IA.Ql> 0 has been discussed [ 16,17,43,45 1, Lipson et al. [ 18 ] have recently provided clear evidence to the contrary for transitions to Rydberg states of Xe,. All of the dissociation energies calculated here are between 400 and 900 cm-‘, indicating that the ion cores are most likely either the Xe$ B211,,Zg or C211J,2U electronic states [ 17,42,46 1. If the excited electron for these states of Xe2 has predominantly 4f character, the ion core must have predominantly C 217j,zUcharacter, as the Xe, excited state must have g symmetry. However, as was discussed above, there are a number of 6d and 8s states of atomic Xe in this energy region. States of Xe, with g symmetry and even 1Rydberg electrons must have ion cores with g symmetry. Configuration interaction will mix (C211,,,,)nf states with (B 211,,2.&s and nd states, and the dissociation energies will be affected accordingly. The irregularity of the vibrational progression gives some evidence for such interactions.
4. Conclusions The two-photon ionization spectrum of Xe2 in the region of the first ionization threshold displays a sharp onset for direct ionization as well as autoionizing resonances corresponding to at least four different electronic states. The onset for direct ionization is significantly higher than in the single-photon ionization spectrum recorded by Ng et al. [ 5 1. Assuming that the earlier measurement is correct, we must conclude that the two-photon ionization cross section at the adiabatic ionization potential is small,
12January 1990
although the distinct step in the two-photon spectrum would indicate that the cross section does not decrease smoothly towards the adiabatic threshold. Analysis of the autoionizing resonances indicates that the observed electronic states most likely have Xe: C 2113i2Uion cores, although configuration interaction with states having B 21’13,29 ion cores may be responsible for irregularities in the spectrum. The effects of such configuration interaction may become far more pronounced if the spectrum can be extended to the B 2113,28and C21JJ,2,, ionization thresholds. Such studies will require higher laser power because of the low intensities of these series at high principal quantum number. Higher power studies should also allow the observation and a more accurate determination of the adiabatic ionization potential.
References [ 1] C.Y. Ng, Advan. Chem. Phys. 52 (1983) 263. [2] P.M. Dehmer and S.T. Pratt, in: Photophysics and photochemistry in the vacuum ultraviolet, eds. S.P. McGlynn, G.L. Findlay and R.H. Huebner (Reidel, Dordrecht, 1985) p. 467. [ 31 P.M. Dehmerand S.T. Pratt, J. Chem. Phys. 77 (1982) 4804. [4] N.Y. Du and C.H. Greene, J. Chem. Phys. 90 (1989) 6347. [5] C.Y. Ng, D.J. Trevor, B.H. Mahan and Y.T. Lee, J. Chem. Phys. 65 (1976) 4327. [6] D.J. Trevor, I.E. Pollard, W.D. Brewer, S.H. Southworth, C.M. Truesdale, D.A. Shirley and Y.T. Lee, J. Chem. Phys. 80 (1984) 6083. [71 C.Y. Ng, D.J. Trevor, B.H. Mahan and Y.T. Lee, .I. Chem. Phys. 66 (1977) 446. [B] P.M. Dehmer and E.D. Poliakoff, Chem. Phys. Letters 77 (1981) 326. [9] P.M. Dehmer, J. Chem. Phys. 76 (1982) 1263. [ lo] XT. Pratt and P.M. Dehmer, Chem. Phys. Letters 87 (1982) 533. [ II] K. Norwood, G. Luo and C.Y. Ng, J. Chem. Phys. 90 ( 1989) 4689. [ 121 G. He&erg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [ 131 W. Gomik, S. Kind& E. Matthias and D. Schmidt, J. Chem. Phys. 75 ( 198 1) 68. [ 141 W. Gomik, E. Matthias and D. Schmidt, J. Phys. B I5 (1982) 3413. [ 15 ] T.D. Raymond, N. Bower& C.Y. Kuo and J.W. Keto, Phys. Rev. A29 (1984) 721. [ 161 P.M. Dehmer, ST. Pratt and J.L. Dehmer, J. Chem. Phys. 85 (1986) 13.
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[ 171 P.M. Dehmer, ST. Pratt and J.L. Dehmer, J. Phys. Chem. 91 (1987) 2593. [ 181 R.H. Lipson, A.R. Hoy and E. Chart, J. Chem. Phys. 90 ( 1989) 4664. [ 191 N. Bowering T.D. Raymond and J.W. Keto, Phys. Rev. Letters 52 (1984) 1880. [ 201 N. Bowering, M.R. Bruce and J.W. Keto, J. Chem. Phys. 84 ( 1986) 709. [ 2 1 ] N. Bower@ M.R. Bruce and J.W. Keto, J. Chem. Phys. 84 (1986) 715. [ 221 J.K. Ku and D.W. Setser, J. Chem. Phys. 84 (1986) 4304. [23] P. Moutard, P. Laporte, N. Damany, J.L. Subtil and H. Damany,Chem. Phys. Letters 132 (1986) 521. [24] P. Moutard, P. Laporte, J.L. Subtil, N. Damany and M. Damany, J. Chem. Phys. 88 ( 1988) 7485. [25] ST. Pratt, E.D. Poliakoff, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 78 (1983) 65. [ 261 R.B. Opsal, KG. Owens and J.P. Reilly, .4nal. Chem. 57 (1985) 1884. [27] P.M. Dehmer and S.T. Pratt, J. Chem. Phys. 75 (1981) 5265. [ 281 C.E. Moore, Atomic energy levels, Vol. 3, NBS Circular No. 467 (US GPO, Washington, 1949). [ 291 A. L’Huillier, L.A. Lompre, D. Normand, J. Morellee, M. Ferray, J. Lavancier, G. Mainfray and C. Manus, J. Opt. Sot. Am. B 6 (1989) 1644. [ 301 R.E. Stebbings and F.B. Dunning, eds., Rydberg states of atoms and molecules (Cambridge Univ. Press, Cambridge, 1983).
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