Photoionization dynamics in CS fragmented from CS 2 studied by high-resolution photoelectron spectroscopy 1

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1 744 Photoionization dynamics in CS fragmented from CS 2 studied by high-resolution photoelectron spectroscopy 1 Anouk M. Rijs, Ellen H.G. Backus, and Cornelis A. de Lange Abstract: The photoionization dynamics of CS have been studied using high-resolution laser photoelectron spectroscopy. The photodissociation of CS 2 at ~308 nm results in highly rotationally excited CS in its X 1 Σ + singlet ground state, as well as in rotationally cold CS in the excited a 3 Π triplet state. The ground-state CS fragments are formed together with sulfur in its 3 P, 1 D, and 1 S electronic states; triplet CS is produced in coincidence with ground-state sulfur ( 3 P). In both channels the photoelectron spectra are dominated by v = 0 propensity, but transitions involving v =1 and 2 are also observed. Key words: photoelectron spectroscopy, photoionization, photodissociation, excited states, reactive intermediates. Résumé : On a étudié la dynamique de photoionisation du CS en faisant appel à la spectroscopie photoélectronique laser à haute résolution. La photodissociation du CS 2 à environ 308 nm conduit à la formation de CS fortement excité d un point de vue rotationnel, dans son état fondamental singulet X 1 Σ +, ainsi que de CS rotationnellement froid dans un état triplet excité a 3 Π. Les fragments CS dans l état fondamental se forment en même temps que du soufre dans les états électroniques 3 P, 1 Det 1 S; les quantités de CS à l état triplet se forment en coïncidence avec le soufre à l état fondamental ( 3 P). Dans les deux voies, les spectres photoélectroniques sont dominés par la propensité v = 0, mais on observe aussi des transitions de v =1et2. Mots clés : spectroscopie photoélectronique, photoionisation, photodissociation, états excités, intermédiaires réactifs. [Traduit par la Rédaction] Rijs et al. 749 Introduction Resonance-enhanced multiphoton ionization, in combination with kinetic energy resolved photoelectron spectroscopy (REMPI-PES), has proven to be an excellent and powerful method to study rotationally resolved photoionization and photodissociation dynamics of rotationally hot photofragments (1 3). In particular, with REMPI-PES, detailed information about the spectroscopy and photoionization dynamics of the intermediate state can be obtained. Fragments formed in a photofragmentation process are often generated with an appreciable degree of electronic, vibrational, and rotational excitation. With sufficient rotational excitation, even for fragments with small rotational constants, the spacings between excited rotational levels in the ion may exceed the energy resolution of the magnetic bottle electron spectrometer used, and rotationally resolved photoelectron spectra can be obtained. These rotationally resolved photoelectron spectra provide information on the photoionization dynamics. In the present work the photodissociation of CS 2, resulting in CS photofragments, has been studied. The ionization energy of CS from its neutral ground state to the lowest X 2 Σ + ionic state amounts to ev (4). The rotational constant of CS + in its ionic ground state is only cm 1 (5). However, when the CS photofragments are formed with sufficient rotational excitation, rotational levels in the ion with high J + are accessed, and rotationally resolved PES can be observed. Carbon disulphide (CS 2 ) has a structure and a photodissociation behavior similar to that of the previously studied N 2 O and OCS (1 3). These molecules all possess a linear ground-state geometry with a 16 valence electron configuration. The absorption spectrum of CS 2 shows many absorption bands between ~100 and 400 nm (6). The photo- Received 7 June Published on the NRC Research Press Web site at on 25 June A.M. Rijs. Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht , 1018 WS Amsterdam, The Netherlands and Laser Centre and Department of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands. E.H.G. Backus. Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The Netherlands. C.A. de Lange. 2 Laser Centre and Department of Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands and Department of Chemistry, The University, Southampton SO17 1BJ, UK. 1 This article is part of a Special Issue dedicated to the memory of Professor Gerhard Herzberg. 2 Corresponding author ( delange@science.uva.nl). Can. J. Chem. 82: (2004) doi: /V04-015

2 Rijs et al. 745 chemistry of CS 2 is strongly dependent on the wavelength of photolysis. Absorption of one photon of 193 nm results in the production of singlet CS (X 1 Σ + ) fragments and sulfur atoms in the singlet or triplet state ( 1 D or 3 P) (7, 8). For photolysis wavelengths towards the ultraviolet, the production of triplet CS (a 3 Π) becomes more important, and between 140 and 125 nm, the CS fragments are almost completely produced in the triplet state (9). CS 2 shows a strong absorption band around nm. This band has been assigned to the 1 ~ ~ X 1 Σ + g transition. This 1 ~ upper state is bent with an SCS angle of 131 and correlates to the 1 u valence state in the linear configuration (10). Onephoton excitation of CS 2 at ~308 nm (4.03 ev) correlates to the transition to the 1 ~ state. CS photofragments are formed by the sequential (1+1) absorption of two photons at ~308 nm (total energy 8.06 ev) via the 1 ~ intermediate state to a predissociative excited state lying near 150 nm (6). This state predissociates, yielding CS photofragments in combination with S atoms. There are several dissociation pathways in our energy region that can be accessed, producing singlet and triplet CS fragments (see Table 1), as follows: [1] CS 2 CS (X 1 Σ + )+S( 3 P J ) [2] CS 2 CS (X 1 Σ + )+S( 1 D 2 ) [3] CS 2 CS (X 1 Σ + )+S( 1 S 0 ) [4] CS 2 CS (a 3 Π)+S( 3 P J ) Dissociation pathways [2], [3], and [4] are spin-allowed processes from the optically prepared state. The first dissociation pathway [1] is a spin-forbidden process, which becomes weakly allowed by the spin orbit interaction associated with the presence of the two sulfur atoms in CS 2. The present paper is concerned with rotationally and vibrationally resolved laser photoelectron spectroscopy of CS fragments. At about 308 nm, the CS photofragments can be produced either in the singlet or the triplet state. The CS fragments formed in the X 1 Σ + ground state are rotationally hot and are studied via a (2+1) ionization scheme using the intermediate B 1 Σ + Rydberg state as a stepping-stone. The intermediate B 1 Σ + Rydberg state represents the lowest member of a Rydberg series converging upon the X 2 Σ + ionic ground state of CS + and is located at about 8.04 ev above the CS (X 1 Σ + ) ground state (11). Together with the CS fragments formed in their singlet state, S atoms are also produced. These sulfur atoms can be formed via dissociation pathways [1], [2], and [3], resulting in ground- ( 3 P J ) and excited-state ( 1 D 2, 1 S 0 ) S atoms. Various (2+1) ionization transitions of S ( 3 P J, 1 S 0 ) are used for calibration. In addition to ground-state CS (X 1 Σ + ) fragments, excited CS fragments can also be formed in the triplet state (a 3 Π) via the dissociation route [4]. The electronically excited triplet CS (a 3 Π) fragments, which possess low rotational excitation, are studied employing a (1+1) excitation step via the intermediate CS ( 3 Σ + ) state, resulting in CS + (X 2 Σ + ) ionic fragments. With our photon energies, the CS (a 3 Π) fragments can only be formed together with ground-state S ( 3 P J ) atoms. The appearance energy of the other dissociation channels of triplet CS (a 3 Π)+S( 1 D 2, 1 S 0 ) lies above our two-photon dissociation energy. Table 1. Threshold energies of the product states in the photodissociation of CS 2. Pathway CS channel S channel Energies (ev) [1] CS (X 1 Σ + ) S ( 3 P J ) 4.46 [2] CS (X 1 Σ + ) S ( 1 D 2 ) 5.61 [3] CS (X 1 Σ + ) S ( 1 S 0 ) 7.21 [4] CS (a 3 Π) S ( 3 P J ) 7.88 Experimental The experimental setup has been reported in great detail previously (1). Briefly, the laser system consists of an XeCl excimer laser (Lumonics HyperEx-460), producing pulsed radiation with a fixed wavelength of 308 nm and a pulse duration of ~10 ns. The laser pulses are generated with a repetition rate of 30 Hz and have an energy of 200 mj pulse 1. The excimer laser output is used to pump a dye laser (Lumonics HyperDye 500), operating on Rhodamine (bandwidth ~0.08 cm 1 ). The dye laser output (about 615 nm) is frequency-doubled in a Lumonics HyperTrak 1000, using a KD*P crystal, resulting in 10-ns pulses with a maximum energy of about 15 mj and a bandwidth of ~0.2 cm 1. The laser light is focused into the ionization region of a magnetic bottle spectrometer by a quartz lens with a focal length of 25 mm. The laser pulse intensity is kept low enough to avoid nonresonant ionization of the parent compound CS 2.Inthe present experiments, the magnetic bottle spectrometer is used in the electron detection mode for both wavelength scans and photoelectron spectra and operates with a collection efficiency of ~50%. Since a time-of-flight method is used, time windows can be selected to achieve optimum resolution. Signal intensities are essentally independent of kinetic energies (1). The CS photofragments are generated in situ by twophoton dissociation of CS 2 (Sigma-Aldrich, HPLC grade). The CS 2 gas sample is effusively introduced into the ionization chamber through a 13 mm outer diameter Pyrex flow tube, leading to a pressure in the ionization region of typically 10 3 mbar (1 bar = 100 kpa). The photodissociation of CS 2 and the subsequent ionization of the CS fragments are performed with the same laser photons of about 308 nm. For calibration of both the wavelength and the photoelectron kinetic energies, well-known resonances of sulfur atoms at two-photon energies of and cm 1 are used (12, 13). Results and discussion At 308 nm, the dissociation of CS 2 produces CS in the X 1 Σ + ground state or in the a 3 Π triplet state. Figure 1 shows a two-photon excitation spectrum of CS fragments in the two-photon energy range cm 1. The spectrum was obtained by monitoring electrons with a kinetic energy of about 0.75 ev (flight time range of ns). These photoelectrons arise from a (2+1) ionization process, starting from the X 1 Σ + (v =1;N ) electronic ground state via the intermediate B 1 Σ + (v =1;N ) Rydberg state to the X 2 Σ + (v + =1;N + ) ionic ground state. The rotationally resolved excitation spectrum shows the rotational distribution

3 746 Can. J. Chem. Vol. 82, 2004 Fig. 1. Rotationally resolved (2+1) REMPI excitation spectrum with electron detection of the Q branch of the B 1 Σ + X 1 Σ + transition of hot CS fragments in the two-photon energy range cm 1 (vacuum energies). Fig. 2. (a) Rotational fit of the measured line positions for the B 1 Σ + (v = 1) Rydberg state, based on the rotational constants of the X 1 Σ + ground state of CS (v = 1) (17). Results are given in Table 2; (b) Residual plot, the difference between the experimental and calculated line positions. of the Q branch ( N =0)ofCSintheB 1 Σ + (v = 1) state, extending from N =35uptoN = 71. The comb above the spectrum in Fig. 1 indicates the assignment of the N rotational levels. In the absorption step, only the transition from the v =1 levelofx 1 Σ + to the v = 1 level of B 1 Σ + is observed. No transitions arising from X 1 Σ + for v 1 were observed. This unusual situation has to do with the properties of the vibrational levels associated with the B 1 Σ + state. It is known from the literature (14) that the B 1 Σ + Rydberg state predissociates, especially from the v = 0 and the v = 2 levels. Moreover, Sapers and Donaldson (15), who studied the rovibrational states of the CS X 1 Σ + fragments formed in the CS nm two-photon photolysis with laser-induced fluorescence detection, observed that CS was formed in its electronic ground state with a bimodal vibrational distribution that peaked at v = 1 with a secondary maximum at v =5.In view of the fact that the CS X 1 Σ + fragments are predominantly formed in their v = 1 vibrational state, and considering the significant predissociation of the v = 0 and v =2 levels of the B 1 Σ + Rydberg state, it is not surprising that we observe the CS X 1 Σ + fragments only in their v = 1 vibrational state. The O and S branches associated with the B 1 Σ + (v =1) X 1 Σ + (v = 1) transition are not observed because of the great difference in line strengths between the Q and the O and S branches (16). The assignment of the peaks in the excitation spectrum of Fig. 1 and the rotational analysis of the Q branch were performed by assigning the measured rotational lines in an iterative fashion. By using the published ground-state rotational constants (17), the B 1 Σ + excited-state rotational constants B were determined by fitting all the rotational energies of the Q branch to the following expression: [5] Q(N ) =v 11 +(B 1 B 1 )N (N +1) (D 1 D 1 )N 2 (N +1) 2 Table 2. Rotational constants (in cm 1 ) for the B 1 Σ + (v =1) Rydberg state of CS (left column), and a comparison with the literature (14) (central column). B 1 Σ + (v =1) B 1 Σ + X 1 Σ + v (36) B (24) D (38) Note: The ground-state rotational constants of CS (X 1 Σ + ) (refs. 17 and 18) are presented in the right column. In our excitation spectrum of rotationally excited CS, we measured only high rotational levels, from N =35upto N = 71. Because we observe these high rotational levels, the centrifugal distortion constant D 1 of the B 1 Σ + (v = 1) state can be determined. The results of the rotational fit are shown in Fig. 2a, and the difference plot between the experimental and calculated line positions is presented in Fig. 2b. The improved rotational constants for the CS B 1 Σ + (v = 1) state are listed in Table 2 together with the CS X 1 Σ + ground-state constants of refs. 17 and 18. The two-photon dissociation of CS 2 at about 308 nm produces, in addition to CS fragments, S atoms in various states (see dissociation pathways [1], [2], and [3]). Several transitions of the S ( 3 P J )ands( 1 S 0 ) fragments are observed following (2+1) ionization. These well-known absorption lines are used for wavelength and kinetic-energy calibration. Dissociation resulting in S ( 1 D 2 ) (route [2]) is not observed because no transitions can be excited via a (2+1) ionization scheme with 308-nm photons. To observe S ( 1 D 2 ) atoms, the photon energy must be shifted to higher energies. Photoelectron kinetic energy scans were obtained by onephoton ionization from the intermediate CS B 1 Σ + (v =1;N ) state into the continua of the ionic ground state X 2 Σ + (v + =1;

4 Rijs et al. 747 Fig. 3. Photoelectron spectrum of rotationally hot CS, produced in the two-photon dissociation of CS 2. The spectrum is obtained by (2+1) ionization, by employing the Q(55) transition via the v = 1 level of the intermediate B 1 Σ + Rydberg state. (a) Inthe spectrum, transitions to X 2 Σ + (v + =1,2,3)areshown;(b) more detailed scan for v =0(v + = 1), showing some rotational structure. Fig. 4. (1+1) REMPI excitation spectrum with electron detection of the 3 Σ + a 3 Π transition of CS photofragments in the onephoton energy range cm 1 (vacuum energies). N + ). Figure 3a presents the photoelectron spectra associated with the Q(55) rotational transition at cm 1 via v = 1. The dominant peak at about 0.74 ev arises from the v = 0 transition. The smaller peaks can be assigned to the v = (v + v ) = +1, +2 transitions. The vibrational v = 0 propensity observed for this X 2 Σ + B 1 Σ + step is expected for onephoton ionization of a Rydberg state. Despite the high rotational excitation of the CS fragment, the ionic spacings are not large enough to measure goodquality rotationally resolved photoelectron spectra. However, if we make a more detailed scan for the v = 0 transition at about 0.74 ev, some partially resolved rotational structure is observed (see Fig. 3b). Besides the Q(55) ( N = 0) feature, peaks are also observed at about mev from the Q(55) peak. These peaks can be assigned to N = ±2 transitions. The rotational spacings between the N = 0 peak and the N = +2 and 2 peaks are expected to be B + (4N + + 6) (24 mev) and B + (4N + 2) (23 mev), respectively. Here B + = cm 1 (5) is the ionic rotational constant, and N + is the rotational quantum number of CS +. Higher order rotational contributions are neglected in this estimate. The rotational structure in the photoelectron spectra of CS is subject to selection rules. For one-photon ionization from the B 1 Σ + (v ) state, the selection rule N + = odd applies, where N = N + N and is a partial wave component of the photoelectron (19, 20). In the present case, the CS Rydberg electron occupies a 3sσ molecular orbital. In an atomic-like picture, ionization of this 3sσ Rydberg electron is expected to lead to p ( = 1) partial waves, and, on this basis, strong N = even transitions would be expected; N = ±1 transitions are forbidden on the basis of these selection rules. However, with the present quality of the rotationally resolved photoelectron spectra, no firm conclusions can be drawn. Following two-photon dissociation of CS 2 at 308 nm, the CS photofragments can be formed in the triplet state, resulting in CS (a 3 Π)+S( 3 P J ). Figure 4 shows the one-photon excitation spectrum in the energy range cm 1, corresponding to the CS fragment produced in its a 3 Π state. The features observed belong to the 3 Σ + (v =0) a 3 Π (v = 2) transition (21, 22). The origin (v 00 ) of this transition lies at 282 nm, whereas we excite with ~300 nm. Ono and Hardwick (21) observed three absorption bands at 282, 291, and 300 nm. These bands are assigned to the 3 Σ + (v =0) a 3 Π (v = 0, 1, 2) transitions, respectively. In our experiment, we have studied only the 3 Σ + (v =0) a 3 Π (v =2) transition. Since the same photons are used for the photodissociation of CS 2 and for the spectroscopy of CS, the other transitions could not easily be accessed in a one-color experiment. Our rotational assignment is based on the results of

5 748 Can. J. Chem. Vol. 82, 2004 Fig. 5. Photoelectron spectrum of CS (a 3 Π), resulting from the (1+1) photoionization via the 3 Σ + Rydberg state. Ono and Hardwick (21). The complexity of the rotational structure that is evident from Fig. 4 is typical for a 3 Σ + 3 Π transition between the two states described in Hund s case (b) (23). Because of the high threshold energy of 7.88 ev for the formation of CS (a 3 Π) (see Table 1) and the vibrational excitation (v = 2), less energy is available for excitation of the internal degrees of freedom, resulting in rotationally cold (N = 4 22) CS (a 3 Π) (21). This situation is in sharp contrast to the formation of CS (X 1 Σ + ) with significant rotational excitation. Since CS (X 1 Σ + ) is produced in a (2+1) and CS (a 3 Π) in a (1+1) REMPI experiment, the very different conditions do not allow any reliable conclusions about branching ratios between both channels. Figure 5 presents the experimental photoelectron spectrum obtained for (1+1) photoionization of CS (a 3 Π, v =2)via the CS ( 3 Σ +,v = 0) Rydberg state at cm 1, producing the X 2 Σ + (v + ) ionic ground state of CS +. The dominant peak in the photoelectron spectrum at about 0.60 ev is the v = 0 transition. The neighboring peaks at the left-hand side of the spectrum arise from v = +1, +2 transitions. No rotationally resolved features are observed in the photoelectron spectrum of CS (a 3 Π, v = 2) because of the low rotational excitation and the small rotational B + constant. The shoulder on the right-hand side of the v = 0 peak is the result of nonresonant multiphoton ionization of 1 S sulfur atoms. In summary, the photodissociation of CS 2 produces CS molecules both in the singlet ground state and in a triplet excited state. The ground state (X 1 Σ +, v = 1) formed is highly rotationally excited, while the triplet state (a 3 Π, v =2)generated is rotationally cold. Comparing the results of the photodissociation of CS 2 with previous studies of the photodissociation of the similar molecules N 2 O (2) and OCS (3), the photodissociation of these molecules produced rotationally hot N 2 and CO photofragments and O and S atoms in their 1 D 2 singlet excited states. In the photodissociation of CS 2 we observe both similarities and differences. In the case of photodissociation of N 2 O and OCS, the N 2 and CO photofragments are only formed in their electronic ground states, and nearly all them are formed in their ground vibrational states (2, 3). The photodissociation of CS 2 produces both ground-state and excited-state CS fragments and results in vibrationally excited CS species, both in the singlet electronic X 1 Σ + ground state and in the electronically excited triplet a 3 Π state. In the photodissociation process of OCS and N 2 O, the C O and the N N fragments appear to act solely as spectators, while in CS 2 the nonbonding electrons of both S atoms contribute to the absorption and both C S bonds are excited and stretched (24). Ultimately, if one C S bond breaks, the other is strongly vibrationally excited. The photoelectron spectra of ground-state CS photofragments are dominated by v = 0 propensity, as was the case with the photoelectron spectra of ground-state N 2 and CO. However, the photoelectron spectra of N 2 and CO could be measured with excellent rotational resolution, whereas in the case of CS it appears more difficult to obtain rotationally resolved photoelectron spectra. This is mostly because the rotational constant of the CS + ion is approximately half that of N 2 + or CO +. Finally, the fact that our experiments on CS, both in its singlet and triplet electronic states, have been performed in the ~1 T field of a magnetic bottle spectrometer should not affect the observed dynamics. Systematic studies have shown that possible magnetic field effects in this type of spectrometer may only be significant for high Rydberg states close to the ionization threshold (25 27). Conclusions The photodissociation dynamics of CS 2 at about 308 nm encompasses a singlet and a triplet CS dissociation channel. The photodissociation via the singlet channel produces rotationally hot CS (X 1 Σ + ) fragments in their first vibrationally excited level. A rotational distribution of CS extending from N =35uptoN = 71 (with a maximum at N = 58) has been observed and assigned. For the first time, a (2+1) high-resolution partly rotationally resolved excitation spectrum of CS (X 1 Σ + ) via the Rydberg level B 1 Σ + has been recorded. Improved spectroscopic rotational parameters have been obtained for the CS (B 1 Σ + ) state. The other dissociation channel of CS 2 at 308 nm results in the production of CS (a 3 Π)initsv = 2 vibrational level, with low rotational excitation. In both cases the photoelectron spectra were dominated by v = 0 propensity, with much weaker transitions involving v = 1 and 2. Acknowledgements AMR acknowledges the Holland Research School of Molecular Chemistry for a Ph.D. fellowship. CAdL acknowledges very useful discussions with Professor J.M. Dyke and members of his group during a stay in Southampton in the context of the EC Network on Reactive Intermediates. References 1. A.M. Rijs, E.H.G. Backus, C.A. de Lange, N.P.C. Westwood, and M.H.M. Janssen. J. Electron Spectrosc. Relat. Phenom. 112, 151 (2000). 2. A.M. Rijs, E.H.G. Backus, C.A. de Lange, M.H.M. Janssen, K. Wang, and V. McKoy. J. Chem. Phys. 114, 9413 (2001).

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