Vapor Phase Electronic Absorption and MCD Spectra for Dimethyl Sulfide, Dimethyl Selenide, and Dimethyl Telluride in the UV Region

Abstract

Electronic absorption and magnetic circular dichroism spectra for (CH 3 ) 2 X, X ) S, Se, and Te, in the vapor phase were measured in the range 2.5-5.3 µm -1 with a spectral bandwidth of e0.003 µm -1 . The spectra show well-resolved vibrational structure for the n X f (n + 1)p Rydberg system with positive MCD B terms for (CH 3 ) 2 Se and (CH 3 ) 2 Te, but a negative B term for (CH 3 ) 2 S. The difference in the MCD spectra is interpreted in terms of the influence of magnetic coupling with the broad n X f σ* valence transition, which lies rather close in energy to the Rydberg system for X ) S, but is separated to lower energy for X ) Se or Te. The interpretation requires a n X f σ* transition assignment of 1 A 1 f 1 B 1 and provides support for the most recent 1 A 1 symmetry assignment of the n X f (n + 1)p Rydberg excited state. The assignment of several weak, previously unreported, bands for X ) Se and Te is also given.

Photodissociation of CD3SCD3 on the First Absorption Band: Translational and Internal Energy Transfer to the CD3 Fragment Studied by Resonant Multiphoton Ionization and Time-of-Flight Spectrometry

“…The spectrum reproduces closely the features observed in higher resolution measurements by other groups. 1,9,10,12,30 According to recent investigations based on magnetic circular dichroism, 9 the absorption of the molecule in the 214-230 nm range is assigned to the first dipole allowed transition 1 1 B 1 (9a 1 r 3b 1 ) r X 1 A 1 , whereas at wavelengths shorter than 214 nm is associated with the more energetic transition 2 1 B 1 (10a 1 r 3b 1 ) r X 1 A 1 .…”

“…This value, which is very close to the limiting value -1, indicates that the dissociating absorption of CD 3 SCD 3 at 229 nm is predominantly associated with a pure perpendicular dipole electronic transition, and hence the observed anisotropy parameter is consistent with the previous assignment of the transition 1 1 B 1 r X 1 A 1 as being responsible for the first absorption band of DMS. 1,[9][10][11][12] Figure 4 depicts the best-fit CM recoil energy distribution P(E rec /E max ) (eq 6) of the CD 3 (V)0) products. The dashed curves in the figure denote distributions that still provide a reasonable fit to the three TOF spectra of Figure 3 and illustrate the uncertainty that affects the present determination.…”

“…In fact, as pointed out in section IVC, the energy disposal into product translation is larger at 229 nm than that found in previous experiments at 193 nm. 6,7 The higher complexity of the excited states accessed at 193 nm, presumably mainly a 1 A 1 state, 9 allows for a more effective coupling to the internal energy of the fragments in comparison with the more direct dissociation process which takes place on the first absorption band.…”

Use of the spatial phase of a focused laser beam to yield mechanistic information about photo-induced chemical reactions

“…The primary excitation step for wavelengths >190 nm is the promotion of a non-bonding sulfur electron (from 3b 1 , the highest occupied molecular orbital) to a 4s Rydberg-like orbital (9a 1 ) or a valence-like C-S anti-bonding (6b 1 ) orbital [23,24] 3 . At the shorter wavelength used in the present study (177 nm, corresponding to ω 3 ), higher-energy Rydberg orbitals, such as the 5s(a 1 ), 4p(b 1 ), 3d(a 1 ) and 3d(b 1 ), are populated [25], and presumably higher lying conical intersections involving the C-S reaction coordinate provide efficient paths to form the same products.…”

Photodissociation Dynamics and Stereodynamics of Methyl Mercaptan and Dimethyl Sulfide from the Second Absorption Band at 201 and 210 nm

“…The eect of non-adiabaticity on photoabsorption is for DMS, as for the rest of doublesubstituted suldes, even more dramatic. The two low-lying electronic states in these systems, 1 1 B 1 and 1 1 A 2 , are energetically accessible from the groundX 1 A 1 state 3,4 although only the 1 1 B 1 ←X 1 A 1 transition is electric-dipole allowed in the limit of C 2v symmetry.…”