G.W. King

Two-photon sequential absorption spectroscopy of iodine-127 and 129 in the 5 eV region

Abstract The two-photon sequential absordtion of iodine to electronic states in the 5 eV energy region has been re-examined using both the 127I2 and 129I2 isotopic species. The vibrational and rotational constants for the E excited state have been determined and an RKR potential curve constructed. Further information on other excited states in this region has been obtained, and the isotope effect used to locate electronic origins.

Two photon sequential absorption spectroscopy of the iodine molecule

Abstract Two independently tunable high-resolution dye lasers, excited simultaneously by a pulsed nitrogen laser, have been used to examine two-step sequential absorption process for I 2 . One laser was used to pump selected rotational levels of the B(O u + ) excited state. The other laser was scanned to probe for transitions from these levels to higher excited states in the 5 eV energy region. Five vibrational progressions were identified and analyzed. Vibrational and rotational constants were obtained for the five excited electronic states, which have ion-pair character. The symmetries of two of these states could be identified as 0 g + ; the other three are either 0 g + or 1 g + .

Ethynylbenzene analysis of the 2790 Å absorption system

Abstract The 2790 A absorption system of ethynylbenzene (phenylacetylene) and three deuterated isomers has been analyzed. The type B origin bands are at 35879.0/35892.0/36034.0/36047.1 cm−1 for C6H5CCH/C6H5CCD/C6D5CCH/C6D5CCD. The transition is A 1 B 2 ← X 1 A 1 under the C2v point group, the molecule remaining planar in the excited state. Most of the intensity is due to forbidden components based on type A false origins separated by single quanta of b2 vibrations from the true origin, of which the band 3501 at 36370.8/36395.6/36511.8/36542.8 cm−1, the strongest band in the spectrum, is typical. The forbidden component borrows intensity from a 1 A 1 ← X 1 A 1 transition. Rotational band contour analyses show that only small changes in geometry occur on electronic excitation. The transition is primarily localized on the benzene ring and is the analogue of the 2600 A system of benzene; the (mean) increase in ring bond lengths on electronic excitation is very close to that found for the benzene transition.

Ethynylbenzene; The vibrational spectra of some deuterated isomers

Abstract The infrared and Raman spectra of para, meta- and ortho-diethynylbenzene and six deuterated isomers of these molecules have been measured. Comparative analyses of the spectra have enabled the 42 normal frequencies of vibration of each molecule to be assigned.

The ultraviolet absorption of cyanogen halides: Part I. Identification and correlation

Abstract The absorption spectra of the three cyanogen halides ClCN, BrCN, and ICN, have been examined under high resolution down to 1250 A. Apart from shifts to longer wavelengths in the order I > Br > Cl, the spectra of the three molecules have basically similar structure, and can be analyzed as follows: (1) weak continuous absorption in the 2600-1800 A region (the A system) resulting from an A1Π ← X1Σ+ transition, the upper state being formed by a π → σ ∗ electron promotion localized on the halogen atom; (2) a second weak continuous absorption at shorter wavelengths (the α system), resulting from transition to either a second 1Π state ( π → σ ∗ ) or a bent state of 1A′ or 1A″ symmetry ( π → π ∗ ); (3) intense discrete absorption in the 1700-1300 A region, containing the B and C systems which are the first members of Rydberg series obtained by π → (n + 1)sσ promotion of a halogen-localized electron; (4) Rydberg bands involving transitions to higher pσ and pπ orbitals.

Rydberg states of carbon dioxide and carbon disulfide

Abstract The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and ( np σ) and ( np π) Rydberg series have been assigned. For both molecules, the lowest excited 1 Π g and 1 Π u states are identified as Rydberg states. The lowest 1 Σ u + state is mainly Rydberg for CO 2 , but contains some valence character for CS 2 , There is no evidence for transitions to additional valence states of these symmetries. It is shown that LCAO MO predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO 2 and CS 2 predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.

The ã3A2 ← X̃1A1 absorption spectrum of thioformaldehyde: A vibrational and rotational analysis

Abstract The results of a vibrational and rotational analysis of the banded a 3 A 2 ← X 1 A 1 transition in CH 2 S CD 2 S are presented. Only three of the six vibrational modes are active in the spectrum with ν′ 2 = 1320 1012 , ν′ 3 = 859 798 , and 2ν′ 4 = 711 516 cm −1 . The spin forbidden transition gains intensity primarily by a mixing of the 1 A 1 (π ∗ ,π) and 3 A 2 (π ∗ ,n) states. This is confirmed by a rotational analysis of the 000 band of both isotopes. The rotational analysis shows that the coupling in the a 3 A 2 state is near Hunds case b and that the spin constants are nearly 10 times greater than those observed for CH2O. A CNDO 2 calculation shows that this difference is due to the greater spin orbit coupling of S in CH2S and to the smaller energy differences between the B 1 A 1 (π ∗ ,π) , b 3 A 1 (π ∗ ,π) , X 1 A 1 , and the a 3 A 2 (π ∗ ,n) states. The r0 structure calculated from the rotational constants is r CS = 1.683 A , r CH = 1.082 A , βHCH = 119.6°, and α (out of plane) = 16.0°. A simultaneous fit of the vibrational levels in ν′4 of CH2S and CD2S to a double minimum potential function yielded a barrier to molecular inversion of 13 cm−1 and an equilibrium out-of-plane angle of 15°.

Cyclopentanone: Inversion doubling in the 3300Å 1A2-X1A1 absorption system

Abstract Analysis of the 3300Aabsorption systems of cyclopentanone and two of its deuterated isomers show that in the excited state, the carbonyl bond length has increased to 1.35Afrom its value of 1.24Ain the ground state, and that the oxygen atom is bent out of the C 5 -C 1 -C 2 plane, producing an inversion doubling of the carbonyl out-of-plane vibrational mode in the excited state. The out-of-plane angle is ∼32° from band contour analysis, and 35° at the potential minima from a fit to the calculated eigenvalues for a Gaussian-type function. The barrier to inversion of the carbonyl group in the excited state is ∼700 cm −1 for all three isomers. The absorption system is due to an n → π * , 1 A 2 - X˜ 1 A 1 electronic transition. The electronic origins coincide with bands at 30 514.0/30 487.3/30 460.5 cm −1 for cyclopentanone, the α-α-α′-α′-d 4 , and the d 8 isomers respectively. It is believed that the transition acquires intensity by an additional vibronic path compared with the analogous transition in formaldehyde. This path involves excitation of the ring-puckering mode, whose frequency increases from 236/220/189 cm −1 in the ground state to 284/268/264 cm −1 in the excited state.

Oxalyl halides: Part II. Vibrational spectra and assignments for oxalyl fluoride and oxalyl chloride fluoride

Abstract The previously unreported infrared and Raman spectra of oxalyl fluoride and the infrared spectrum of oxalyl chloride fluoride have been measured and analyzed. An assignment has been made for all twelve fundamental frequencies of the fluoride and for eleven fundamentals of the chloride fluoride. The spectra of both molecules can be explained satisfactorily on the assumption that they are present in the trans planar form only.

Thioformaldehyde: Vibrational analysis of the Ã1A2-X̃1A1 visible absorption system

The electronic absorption spectra of thioformaldehyde and thioformaldehyde-d2 have been obtained. A vibrational analysis of the discrete band system in the 6100-4400-A region is reported. The type A origin bands are at 16 39416 484 cm−1 for CH2SCD2S, and are magnetic dipole allowed. The electronic transition is A1A2-X1A1 under the C2v point group. Most of the intensity of the system is in type B bands, and is due to vibronic mixing with higher 1B2 states when the inversion mode ν4 is excited. The molecule in the excited 1A2 state is “floppy-planar,” having a broad potential function with a barrier of the order of 20 cm−1 to the inversion motion.