Multiphoton ionization spectroscopy the neutral S1←S0 transition and the ionic state of p-xylene

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MULTIPHOTON IONIZATION SPECTROSCOPY THE NEUTRAL SpSO TRANSITION AND THE IONIC x STATE OF pXYLENE IS. WALTER, K. SCHERM and U. BOESL Institutfir Physikalischeund TheoretischeChemie der TechnischenUniversitdtMiinchen,Lichtenbergstrasse 4, D-8046 Gurching,FederalRepublicof Germany Received 14 June 1989; in final form 14 July 1989

We present au assigned spectrum of the frost electronic transition of pxylene observed by resonanceenhanced multiphoton ionization in a time-of-flight mass spectrometer. The resulting vibrational populations in the ionic ground state due to different neutral vibronic intermediate S, states have been investigated by means of time-of-flight photoelectron spectroscopy. This allows a check of the propensity rules for photoionixation of this molecule. The spectrum shows a rich structure. and some vibrational frequencies of the ion are determined.

1. Introduction In the last two decades much work has been done on the electronic spectroscopy of benzene and its derivatives. For many substituted benzenes, like toluene [ 1,2] and aniline [ 3 1, the lowest electronically excited singlet state (S, ) has been investigated thoroughly and assigned vibrationally resolved spectra are available. For the S, state of pxylene, however, less vibrational information is available. There exists an absorption spectrum of the solid phase [ 41, some theoretical work [ 5 1, and ( l-t- 1) photoionization spectra over a range of 1400 cm- ’ [ 61. Recently the S, +S,, transition of m-, o-, pxylene in the gas phase has been studied by ( 1+ 1) photoionization and extensively by ( 2 + 2 ) photoionization [ 7 1. Almost nothing is known about the vibrations of the molecular ion. Only a He( I)-photoelectron spectrum of m- and pxylene [ 81 with electronic state resolution exists. On the other hand for benzene [ 9 1, halogenated benzenes [ lo], toluene [ 111, and aniline [ 121 vibrationally resolved multiphoton ionization photoelectron spectra of the ionic ground states have been published. In this work we present a spectrum of the electronic transition S, cSO of pxylene over a region of 3800 cm-’ and an assignment of the vibronic structure. Efficient rotational cooling by supersonic ex-

pansion leads to sharp peaks. For five intermediate vibronic states in the S, electronic state of the neutral molecule, we investigated the resulting vibrational population in the ground state of the molecular ion by means of time-of-flight photoelectron (TGF PE) spectroscopy. Although in all cases the most intense signal corresponds to the Au=0 transition in the ionization of the excited singlet state, intense excitations of other vibrations were also observed. An assignment is given and some vibrational frequencies of the molecular ion are reported.

2. Experimental The resonance enhanced multiphoton ionization spectrum was generated in a Reflectron time-of-flight (RETGF) mass spectrometer. This apparatus has been described in detail elsewhere [ 131. Briefly, we used a pulsed, skimmed supersonic jet with a backing pressure of 2 bar of Ar as carrier gas and = 8 mbar pxylene. The beam of an excimer- (EMG 102E) pumped frequency-doubled dye laser (FL 2002) crossed the molecular beam in an electrical accelleration field. The generated ions were mass selected in our RETOF and the intensity of the xylene signal with a mass of 106 dalton was measured as a function of the laser wavelength using a gated integrator. The UV

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intensity was kept low enough to avoid fragmentation. The experimental apparatus for the multiphoton ionization TOF PE spectroscopy consists of a vacuum chamber and a field free drift tube surrounded by magnetic shielding. Xylene seeded in Ar was introduced into the apparatus by a pulsed nozzle. The ionization was performed by crossing the molecular beam with UV light of the appropriate wavelength generated by the pulsed, frequencydoubled dye laser. The photoelectrons drifted 52 cm from the ionization region to a microchannel plate detector. Thus the time of flight of the electrons was inversely proportional to the square root of their kinetic energy. The signal was digitized via a LeCroy TR8828C transient recorder with a sampling rate of 200 Msamp / s and then processed with a VMEbus computer system. The time of flight of the electrons was converted to electron kinetic energy; with the energy of the ionizing photons the ion internal energy scale is known. The TOF PE spectra were reproducable to within 10 meV. By comparison with TOF PE spectra of benzene and toluene (molecules with a well known ionization potential) recorded by us and others [ 8,111 our error margin on the absolute energy was determined to be = 20 meV. By observing the signal intensity with respect to the kinetic energy of the electrons one obtains information about energy levels and their population in the ion. The ionizing laser had a beam diameter of 2 mm; it was not focused in order to avoid space-charge effects even for an ion yield of up to some lo4 ions per laser pulse. (Due to the geometry of our apparatus only one electron in lo4 produced is detected.) Typical UV pulse energies were less then 50 p.I. Reducing the power and/ or focusing of the laser did not improve our resolution. The spectra were averaged over 10000-40000 laser pulses with a repetition rate of 20 Hz. For our relatively high energy electrons ( !Z600 meV) the jitter of the transient recorder of f2 channels limits the resolution to 25 meV. 3. Results and discussion 3.1. The S,+S, transitionofp-xyiene Assuming the DZh point group, the molecular ground state So has pie symmetry and the first excited 474

29 September 1989

electronic state S1 has BZusymmetry. Thus the electronic transition S1c% is dipole-allowed and a strong origin is observed in the one-photon spectrum. The transition was investigated by means of one-photon resonance two-photon absorption. The S, origin is located at an energy which is more than half the ionization potential. The measured spectrum (ranging from 36350 to 40150 cm-‘) and the vibronic assignments are shown in fig. 1 and table 1. We use here the Wilson notation [ 14 1. The spectrum is dominated by progressions of v7a, which is essentially a C-CH3 stretching mode of a, symmetry with a frequency of 1186 cm-‘. For this variation an anharmonicity of 1.5 cm- ’ was observed. The progressions are built up on the origin and on nearly all other observed vibrations. The following vibrational modes could be assigned: (i) vl which is a radial skeletal or ring breathing mode of a, symmetry. All peaks involving a vI vibration show a splitting of 28 cm-’ probably due to a Fermi resonance with an unknown combination state of a, symmetry; the 6a2 state may be a candidate. The frequencies of the resulting two components are 773 and 801 cm-‘. This behaviour is well known for methylsubstituted benzenes like toluene and higher alkylsubstituted benzenes [ 1,2]. (ii) vg. which is also a radial skeletal vibration of a, symmetry; the frequency was determined as 424 cm- ‘. (iii) v&,which is a radial skeletal vibration of bSI symmetry with a frequency of 553 cm-‘. In this vibration the methyl substituents are not strongly involved, thus the frequency is similar to that in benzene. (iv) v9, which is a C-H in plane bending mode of a, symmetry with afrequencyof937 cm-‘. (v) ~,whichisanin-plane bending mode of b3, symmetry strongly involving the methyl groups. The frequency was determined as 369 cm- ‘. The frequencies of the combination vibrations correspond almost perfectly to the sums of the frequencies of the vibrations involved, only for the combinations vgb+ n viaand vg, + n vTa,n = 1,2, is the frequency reduced by 12 and 5 cm-‘, respectively. The internal rotation of the methyl groups could also be observed, alone and in combination with the vibrations IQand v7a (see fig. 1). We determined an energy of 53 cm-’ for excitation of Oa;3a’,’in the S1 state, in fair agreement with the value obtained by Breen et al. [ 15 1. For a detailed investigation of this internal rotation see refs. [ 15,16 1. No hotbands could

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29 September 1989

-1:

7at

-6bA la,” -baA la;

‘I-9bA la; --

’T

9al,7a”,

\

1

38000

37000 -

40000

39000

FREQUENCY [cm-‘]-

Fig. 1. Spectrum of the S,+S, transition in pxylene, observed by resonance-enhanced two-photon ionization, and the vibronic ass@ment. An asterisk indicates excitation of the internal rotation in the SI state. Table 1 Peak positions of assigned transitions Frequency (cm-‘)

Assignment

Frequency (cm-l)

36732 36785 37101 37156 37285 37505 37533 37558 37588 37699 37917 37969

0;

38298 38341 38468 38686 38715 38849 39099 39480 39522 39865 39895 40029

be identified. 1.

int. rot. 9bh 64 6b; l:, 1; +int.rot. 94 74 74 + introt.

Assignment

lb7d 9476 74

9bb7a$ 6474

in a one-colour experiment by the absorption of two photons, the first photon being in resonance with a vibrational state in the neutral S, electronic state. We derived vibrational frequencies for the ionic ground state and the population of these vibronic levels from ionic spectra resulting from ionization of five different intermediate states: The vibrationless S1 and the vibronic states 9b1 and 6b’ are both components of 1’. The frequencies of the S, &So transitions and the available excess energies above the ionization threshold are given in table 2. The resulting PE speo tra are shown in figs. 2 and 3. For comparison the

1;7g 9d7a!J

The peak positions are listed in table

3.2. The ionic ground state of pxylene The xylene ions were investigated by multiphoton ionization TOF PE spectroscopy. Xylene is ionized

Table 2 Excess ion energies, IP: 8.465 * 0.02 eV Transition

Frequency (cm-‘)

Excess energy (meV)

G 9% 6bh

36732 37101 37285 37505 37533

648 740 la6 840 a47

ld

475

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CHEMICAL PHYSICS LETTERS I

29 September 1989

r

13:WimcV

a)

al rll

I

I I I I I

IIf

0:

0:

A=50meV

ll

bl

J

b)

9b;

,

.

*

,

,

cl

cl

6b;

12,

0

I 200 ION INTERNAL

400 ENERGY

I 600

ImeVl

Fig. 2. MUPI-PE spectrum of the ionic ground state of pxylene due to ionization of different vibronic levels of the neutral S, intermediate state. (a) 0: (S,t!&) transition; (b) 9bJ (S,+) transition; (c) 6bb (S, 4,) transition.

PE spectrum arising from ionization via the O$(S1-SO) transition is shown in both figures. We determined the ionization potential to be 8.465 f 0.02 eV. Literature values vary from 8.37 to 8.80 eV [ 171. Some features are common to all observed PE spectra:

(A) There is a strong progression with a spacing of about 50 meV ( z 400 cm- ’). In the spectrum resulting from ionization via the 0: ( S1c S,) transition in particular, this progression can be observed up to a quantum number of 11. From general vibrational 476

i

n

2io

ION INTERNAL

s ENERGY

L;IO

GO

ImeVl

Fig. 3. MUPI-PE spectrum of the ionic ground state ofpxylene due to ionization of different vibronic levels of the neutral S, intermediate state. (a) a (S,&,,) transition; (b) lb(S,tS,,) transition 773 cm-’ component; (c) 1; (S,+&) transition 801 cm-’ component.

selection rules for the ionization process [ 181 this vibration should be of a, symmetry. The only a, mode with appropriate energy is & (frequency in the S1 state: 424 cm-‘). There are, however, many examples where the vibrational population in the ion does not follow this selection rule. (See e.g. refs. [ 10,111. ) We assign this vibration to Ygbfor two reasons: ( 1) The frequency of 400 cm-’ is similar to that of &,b in the S, state (369 cm-‘). (2) By ionizing via the v,(S, ) intermediate state the most intense signal

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should be due to excitation of the Vgbmode in the ion (assuming a Au=0 selection rule). In the corresponding PE spectrum the first and strongest peak is observed at an internal energy which is 55 meV above the IP. This corresponds exactly to the energy of the fust progression (second peak) in the PE spectrum obtained with the 0: (S,cSo) as neutral intermediate transition. (B) There is a further vibrational spacing with a frequency of about 195 meV (a 1570 cm-‘). This is nearly the energy of four quanta of the up!,mode, so these peaks are superimposed. Because of the characteristic intensity of this signal, which can be seen in all our PE spectra, we assume it to be an independent vibration and assign it tentatively to usa (the C-C stretching mode of a, symmetry; frequency in the So state: 1581 cm-’ [ 191). Because there is evidence that the first intense peak corresponds to a Au=0 transition from the S1 state to the ion, excitation of V6bin the ion can be seen as the strongest signal in the PE spectrum due to ionization with the V6b(S1tS0) transition. The frequency was determined as 75 meV (~600 cm-‘; frequency in the S, state: 553 cm- I). In the same way the frequency of the v, mode was determined as about 115 meV ( a930 cm-‘; frequency in the SI state: 773 and 801 cm-‘). Table 3 lists all observed peaks with their assignments.

Table3 Vibrationalenergies of the ionic ground state (in meV) Vibration

Transition for ionization 0:

9b;

6b;

1’

8a’ 9b’ 9b2 9b’ 9b’ 0’q 9b6 9b’ 9b8 9bg 9b’O 9b”

197 54 96 148 197 247 295 347 394 453 493 537

192 55 110 148 204 247 284

197 53 100 153 197

4. slllumary We have presented new spectroscopic data on an important organic molecule and its cation, namely p xylene. The electronic S, t So transition in neutral p xylene was investigated and the vibronic structure assigned. The frequencies of a number of vibrations were determined. Vibrational structure could also be observed for the pxylene ion and vibrational frequencies of the ionic ground state have been determined. Vibrational populations resulting from excitation of different neutral intermediate states during the twophoton ionization have been investigated. A Au=0 propensity was found, but intense excitation of other vibrations could also be observed. Thus this is not a very stringent rule. In particular ugb and vgb ap peared very intense in our PE spectra. These pop ulation distributions yield valuable information for further studies of the pxylene ion such as spectroscopy of electronically excited ionic states or ion kinetics.

Acknowledgement The authors wish to thank Professor Dr. H.J. Neusser for the generous loan of the photoelectron spectrometer and professor Dr. E.W. Schlag for his continuous interest and helpful discussions.

References 1:

75

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113,119 189,195 55, 54 91, 85 150,152 189,195

[ 1] J.B.Hopkins,D.E. Powers and R.E. Smalley, J. Chem. Phys. 72 (1980) 5039. [2] R.Vasudev and J.C.D.Brand,Chem.Phys.37 (1979)211. [3] D.A.Chemoff and S.A. Rice, J. Chem. Phys. 70 (1979) 251 I, and references therein. [ 41 V.L. Broude et al., Absorption spectra of molecularcrystals, Science Council Kiev ( 1965). [ 51M.V. Prijutov, Opt. Spectry. 32 ( 1972) 32. [6] T. Ebata, Y. Suzuki, N. Mikami, T. Niyashi and M. Ito, Chem. Phys. Letters 110 (1984) 597. [7]T.G. Blease, R.J. Donovan, P.R.R. Langridge-Smith, T. Ridley and J.P.T. Wilkinson, in: Resonance ionization spectroscopy 1986, Inst. Phys. Conf. Ser. No. 84, eds. G.S. Hurst and C.G. Morgan, p. 217; T.G.Blease,R.J. Donovan, P.R.R. Langridge-Smith and T. Ridley, Laser Chem. 9 (1988) 241;

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T. Koenig, M. Tuttle, R.A. Wielesek, Tetrahedron Letters 29 ( 1974) 2537. [8] S.R.Long,J.T. Meek, J.P. Reilly, J.Chem. Phys. 79 (1983) 3206. [ 91 H. Ktlhlewind, A. Kiermeier, H.J. Neusser and E.W. S&lag, in: Resonance ionization spectroscopy 1986, Inst. Phys. Conf. Ser. No. 84, eds. G.S. Hurst and C.G. Morgan, p. 121. [lo] S.L. Anderson, D.M. Rider and R.N. Zare, Chem. Phys. Letters93 (1982) Il. [ 111J.T. Meek, S.R. Long and J.P. Reilly, J. Phys. Chem. 86 (1982) 2809. [ 121J.T. Meek, E. Sekreta, W. Wilson, KS. Viswanathan and J.P. Reilly, J. Chem. Phys. 82 (1985) 1741.

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[ 13 ] U. Boesl, H.J. Neusser, R. Weir&auf and E.W. S&lag, J. Phys. Chem. 86 (1982) 4857. [ 141 E.B. Wilson Jr., Phys. Rev. 45 (1934) 706. [ 15 ] P.J. Breen, J.A. Warren and E.R. Bernstein, J. Chem. Phys. 87 (1987) 1917. [ 161 K. Okuyama, N. Mikami and M. Ito, J. Phys. Chem. 89 (1985) 5617. [ 171 R.D. Levin and S.G. Lias, NSRDS, Ionization potential and appearance potential measurements, 1971-1981 (US Department of Commerce, Washington, 1982). [ 181 J.W. Rabalais, in: Principles of ultraviolet photoelectron spectroscopy (Wiley, New York, 1977) . [ 191 G. Varsanyi, Assignments for vibrational spectra of seven hundred benzene derivatives (I-Ii&r, London, 1974),