A resonance enhanced multiphoton ionization study of the CS2 molecule: The 4p Rydberg states

Abstract

The resonance enhanced multiphoton ionization (REMPI) spectrum of jet-cooled CS2 has been recorded in the one-photon wavelength range 460–500 nm, corresponding to the three photon excitation energy range 60 000–65 000 cm−1. A previous assignment of one photon forbidden transitions in this region to 3d Rydberg states is shown to be incorrect and reassigned to the 4pπ 1,3Δu states. In fact all the observed states in this region can be assigned to 4p Rydberg states; the 4pσ 1,3Πu states at 62 768 and 62 083 cm−1, respectively, and the 4pπ 1,3Δu states at 64 214 and 63 698 cm−1, respectively. Another band at 64 374 cm−1 may be due to a three photon excitation to the 4pπ 1Σ+u Rydberg state. Our resolution is sufficient to resolve band shapes enabling symmetry assignments when coupled with their polarization behavior. The fact that the origin bands are not degraded and that Δν=0 sequence bands are strongly excited whereas Δν≠0 transitions are either absent or very weak implies that the upper states have a linear geometry similar to that of the ground state. A comparison of singlet–triplet splittings suggests the 4pπ Δu states have stronger Hund’s case (c) character than the 4pσ Πu state. While CS+2 was generally the predominant ion formed, resonance ionization through the 1Δu←X 1Σ+g origin band led to an unusual predominance of S+ and CS+ ions. This is explained by multiphoton fragmentation of CS+2 via an accidental one-photon resonance from the X 2Π1/2 (v=0) ionic ground state.

Resonance enhanced multiphoton ionization spectroscopy of carbon disulphide

“…5͑a͔͒ was obtained following excitation at 483.1 nm ͑3ṽϭ62 100 cm Ϫ1 ͒; the kinetic energy of the major peak ͑0.188 eV͒ is consistent with ionization to the v ϩ ϭ0 ͑shorthand notation for v 1 ϩ ϭv 2 ϩ ϭv 3 ϩ ϭ0͒ level of the lower, ͓3/2͔, spin-orbit component of the ion thus implying, by Franck-Condon arguments, that the resonance enhancement also involves an electronic origin, almost certainly that of a Rydberg state built on the ͓3/2͔ ion core. Such a conclusion is fully consistent with that reached in a number of previous analyses; 8,10,13,39 namely, that this feature should be assigned as the origin of the resonance. The associated REMPI-PE spectra shown in Figs.…”

“…1 and 3. In fact, as Baker et al 39 have demonstrated ͑and we confirm but do not show in Fig. 4͒, ion fragmentation can also give rise to local intensity variations in the mass selected REMPI spectrum of CS 2 : The 3ϩ1 REMPI spectrum shown in Fig.…”

“…5͑e͔͒ shows a further example of ionization from an intermediate state which appears to be built on a mixed ion core. This photoelectron spectrum was obtained following 3ϩ1 REMPI at an excitation wavelength of 466.1 nm ͑3ϭ64 369 cm Ϫ1 ͒, a feature that has been tentatively assigned 39 as the origin of the 1 ⌺ u ϩ ←X 1 ⌺ g ϩ transition associated with the 4 p u ←2 g Rydberg excitation. It is dominated by peaks indicative of ion formation in the v ϩ ϭ0 levels of both spin-orbit states of the ion.…”

“…Li et al 37,38 and, more recently, Baker and co-workers 39,40 have investigated the energy region 62 000-65 000 cm Ϫ1 using both one color 3ϩ1 and two color ͑1ϩ1Ј͒ϩ1 REMPI spectroscopy. As a result it is now clear that, in this energy range, the vertical electronic spectrum of CS 2 is dominated by transitions to excited states derived from the Rydberg configuration ͓ 2 ⌸ g ]4p u .…”

“…As we show later, the present REMPI studies, supplemented by parallel measurements of the accompanying REMPI-PE spectra, serve to substantiate, and refine, many of the arguments contained in the latter works. 39,40 Assignments proposed in a number of the earlier studies 5,8 of the Rydberg states of CS 2 have been based on analogy with the all-first-row prototype CO 2 , for which a number of rotationally resolved Rydberg transitions are known and for which a limited range of ab initio calculations have been reported. 41 In the context of the present work it is pertinent to note that recent 3ϩ1 REMPI and REMPI-PES studies of CO 2 have revealed the spin-orbit split…”