Rotational spectrum of SiC2 in the ν3 excited state

The rotational spectra of SiC₂ in the vibrationally excited states of the ring deformation mode (υ₃ = 1, 2) were observed in the frequency region of 140–400 GHz by using a source-modulated microwave spectrometer combined with a free space absorption cell. SiC₂ was produced in the cell by discharging a mixture of SiH₄, C₂H₂ and CO. Least-squares analysis of the observed spectral lines yielded the rotational constants and the centrifugal distortion constants precisely. Sextic, octic and decatic centrifugal distortion constants were required in the least-squares fit in order to get a good fit of the observed frequencies to those calculated within experimental errors. The inertial defects for the υ₃ = 1 state and the υ₃ = 2 state do not show a linear dependence on the vibrational quantum number. The quartic centrifugal distortion constants, Δ_JK, Δ_K and δ_K, are abnormally large, and show a large change on the vibrational states. These abnormal behaviours are interpreted in terms of a large amplitude motion of the ν₃ mode.     

Infrared emission spectrum of HNO: The ν1 band

HNO has been observed in emission from a radiofrequency discharge through a mixture of ammonia and oxygen. The ν₁ (NH stretching) band in the region 2400–3200 cm⁻¹ has been recorded with a Fourier transform spectrometer using an apodized resolution of 0.04 cm⁻¹. The analysis of the band has been extended to higher K values than in earlier work.     

Time-resolved laser-induced fluorescence of the iodine B3π(0u+) state: Study of the second maximum of predissociation

Fluorescence decay lifetimes for specific rovibrational levels of the I₂ B³π(0_u⁺) state, have been obtained by time-resolved laser-induced fluorescence. Gyroscopic and hyperfine predissociation parameters C_v² and a_v², for vibrational levels of the B³π(0_u⁺) state around the second maximum of predissociation, v' = 24, 25, have been determined. Results are discussed in terms of the dependence of the predissociation paramaters on the Franck-Condon densities and the corresponding electronic matrix elements.     

Time-resolved photoelectron studies of the relaxation of the S1(1E′') excited state of sym-triazine

We present photoelectron spectra of the lowest singlet state (¹E′') of sym-triazine obtained with 2 ns and 5 ps excitation sources. Comparisons of picosecond and nanosecond spectra are made at three wavelengths, corresponding to the 6₀¹, 6₀² and the 6₀¹12₀¹ vibrational levels. The spectra show that rapid (</ 5 ps) relaxation to the lowest triplet state occurs subsequent to singlet excitation. Coupling of S₁ levels to the T₁ manifold is very inhomogeneous with intersystem crossing rates ranging from 10⁷ s⁻¹ to > 10¹¹ s⁻¹. Furthermore, for excitation at the 6₀¹12₀¹ level we find evidence for relaxation to another excited state at rates ⩾ 10¹¹ s⁻¹.     

Pseudo-Jahn-Teller effect as the origin of the exalted frequency of the b2u Kekulé mode in the 1(1)B2u excited state of benzene

In this paper we show that a pseudo-Jahn-Teller (PJT) coupling between the (1)A1g ground state and the (1)B2u excited states along the Kekulé mode of b2u symmetry is responsible for the surprisingly low frequency of this mode in the ground state and its remarkable upward shift of 261 cm(-1) upon excitation to the first (1)B2u excited state.     

Theoretical study of the valence π → π* excited states of polyacenes: anthracene and naphthacene

The valence π → π ^* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon spectra and T-T spectra of low-lying valence π → π ^* excited states of anthracene and naphthacene. The present theory predicts the valence π → π ^* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions, but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can be used as a model for understanding the excited states of larger polyacenes. Received: 22 April 1998 / Accepted: 6 July 1998 / Published online: 28 September 1998     

The pure rotational spectrum of the CrS radical in its X 5Π(r) state

The pure rotational spectrum of the CrS radical has been measured in its ground X (5)Π(r) state using gas-phase millimeter/submillimeter direct absorption methods. The molecule was created by the reaction of chromium vapor, sublimed in a Broida-type oven, with hydrogen sulfide. Eleven rotational transitions were recorded for this free radical in the frequency range of 280-405 GHz; in most transitions, all five spin components were observed, and lambda-doubling was resolved in the Ω=0, 1, and 2 ladders. The data were fit with a Hund's case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling constants were established. Higher order spin and spin-orbit terms were essential in the analysis. The lambda-doubling constants indicate a nearby (5)Σ(+) state at an energy of ～1500-2000?cm(-1). A bond length of 2.0781 A? was derived for CrS from the data, which is larger than the value of 2.0682 A? found for MnS by ～0.01?A?. In contrast, the bond distance for MnO is greater than that of CrO by 0.03 A?, an illustration of the subtle differences between 3d oxide and sulfides. CrS is the second molecule in a (5)Π state that has been studied by rotational spectroscopy.     

The emission spectrum of AsH2(Ã2A1 → X̃2B1) via uv photolysis of AsH3

The AsH₂(òA₁ → X̃²B₁) emission spectrum (402–650 nm) following ArF laser photolysis of AsH₃ is reported. Seven bands are assigned and the relation ν = 19928 + (868v'₂-3v'₂²)-(987v″₂-6v″₂²) is obtained. The AsH₂ (òA₁) radiative lifetime is 130 ± 20 ns. Emission spectra from nascent As and a multiphoton ionization signal were also obtained.     

Intramolecular 2pπ* → 3dπ charge transfer in the excited state of phenyldisilane studied by picosecond and nanosecond laser spectroscopy

Intramolecular 2pπ^* → 3dπ charge transfer in the excited state of phenyldisilane occurs very rapidly (<10 ps) both in MP at 293 K and in EPA glass at 77 K. At room temperature a long-lived 425 nm transient (which is assigned to a rearranged intermediate) is produced with a rise time of 30 ps, showing that the transient formation proceeds via the ¹(2pπ,3dπ) CT state.     

The rotational analysis of a Doppler-limited Ã1A″-X̃1A′ fluorescence excitation spectrum of HSiF at 430 nm

A rotational analysis is presented for the type-c origin band of an ùA″-X̃¹A′ electronic transition of HSiF, recorded at Doppler-limited resolution using laser-induced fluorescence. An extension of the equations of Hougen and Watson quantitatively accounts for the intensity of many axis-tilting branches, and the corresponding depletion of the regular type-c branches, with a tilting angle of 2.3°. The bond angle increases from about 98° to 115° on excitation.     

Laser-induced photoionization of Ar2(3Σu+) at 308 nm

Experimental evidence is provided to show that the Ar₂(³Σ⁺_u) excimer is photoionized by absorption of light at 308 nm. This direct photoionization of Ar₂(³Σ⁺_u) was used to measure the distribution of atomic states belonging to the Ar(3p⁵4p) electronic manifold produced in dissociative recombination of Ar₂⁺(²Σ_u⁺) at atmospheric pressure. It was found that electronically excited states, Ar(2p₂) and Ar(2p₁₀), accounted for 96% of the excited state population of the Ar(3p⁵4p) configuration produced in dissociative recombination. The Ar₂(³Σ_u⁺) state is also photodissociated directly at 308 nm producing electronically excited Ar atoms more energetic than the Ar(3p⁵4p) configuration.     

The role of the πσ* state in intramolecular charge transfer of 4-(dimethylamino)benzonitrile

The solvent-polarity dependence and temporal characteristics of the transient absorption of 4-(dimethylamino)benzonitrile, DMABN, and 4-(dimethylamino)benzethyne, DMABE, demonstrate the presence of the πσ*-state absorption at about 700 nm and the ππ* (LE)-state absorption at about 520 nm and 450 nm. The rise and decay times of the πσ*-state transient differ from those of the ππ*-state transients in both compounds. Moreover, the peak position of the πσ*-state absorption is blue-shifted and more intense in acetonitrile as compared to n-hexane, whereas the band positions of the ππ*-state absorptions are essentially the same in the two solvents. For DMABN in acetonitrile, the rise time (～4.3 ps) of the twisted intramolecular charge transfer (TICT)-state transient at 330 nm is identical to the decay time of the πσ*-state transient. The 4.8 ns decay time of the TICT-state absorption of DMABN is longer than the 2.9 ns decay time of the intramolecular charge-transfer (ICT) fluorescence, indicating that the fluorescent ICT state differs from the TICT state observed in transient absorption. These results are consistent with the presence of a low-lying πσ* state in DMABN (and DMABE), and the role the πσ* state plays in the formation of the TICT state of DMABN.     

Solvent-induced shift of the lowest singlet π → π* charge-transfer excited state of p-nitroaniline in water: an application of the TDDFT/EFP1 method

The combined time-dependent density functional theory effective fragment potential method (TDDFT/EFP1) is applied to a study of the solvent-induced shift of the lowest singlet π → π* charge-transfer excited state of p-nitroaniline (pNA) from the gas to the condensed phase in water. Molecular dynamics simulations of pNA with 150 EFP1 water molecules are used to model the condensed-phase and generate a simulated spectrum of the lowest singlet charge-transfer excitation. The TDDFT/EFP1 method successfully reproduces the experimental condensed-phase π → π* vertical excitation energy and solvent-induced red shift of pNA in water. The largest contribution to the red shift comes from Coulomb interactions, between pNA and water, and solute relaxation. The solvent shift contributions reflect the increase in zwitterionic character of pNA upon solvation.     

Excited state N2+(B̃) fragments from the photodissociation of N4+ at 270–290 nm

The photodissociation spectrum of N₄⁺ from 270 to 310 nm is broad and structureless at 300 K, with the cross section diminishing by a factor of 10 towards shorter wavelengths. We have detected fluorescence from N₂⁺(B̃) excited state fragments having quantum yield < 0.01 and risetime < 10 ns. The threshold at λ ≈ 300 nm indicates no exit channel barrier.     

Time-resolved Laue diffraction of excited species at atomic resolution: 100 ps single-pulse diffraction of the excited state of the organometallic complex Rh2(μ-PNP)2(PNP)2·BPh4

The polychromatic Laue technique has been applied in 100 ps delay synchrotron pump-probe experiments of the triplet excited state of a Rh(I) dinuclear complex. The observed contraction of the Rh-Rh distance of 0.154 (13) ? is less than predicted by a series of theoretical calculations, a difference attributed to the constraining effect of the crystal lattice.     

Radiative lifetimes of the B1Π state of LiH and LiD

The lifetimes of the B¹Π vibrational levels of LiH and LiD were measured in a molecular beam apparatus using the method of delayed coincidences. The results for LiH are: τ(υ′ = 0) = 11.4 ns, τ(υ′ = 1) = 17.0 ns, τ(υ′ = 2) = 24 ns and for LiD τ(υ′ = 0) = 11.0 ns, τ(υ′ = 1) = 14.0 ns, τ(υ′ = 2) = 21 ns. These values are in good agreement with theoretical calculations. Three new brans of the A-X system were identified.