Ab initio/empirical potential energy functions and stretching vibrational frequencies of the X̃ 2Π and à 2Π states of the chloroacetylene cation

Anharmonic potential energy functions for the stretching vibrational motions of the two lowest doublet states of chloro-acetylene cation have been constructed using ab initio RHF SCF calculations and some experimental information. Stretching vibrational frequencies are calculated variationally for four different isotopomers. The ν₁ vibrations of H¹²C₂³⁵Cl⁺ are predicted to occur at 3176 (X̃ ²Π) and 3231 (à ²Π) and the ν₂ vibration of the à state at 2081 cm⁻¹.     

The ab initio calculation of the band origin and vibrational frequencies of the Ã-X̃ system of HNCl using the non-rigid bender hamiltonian

A three-dimensional fit of ab initio MRD CI potential data has been made for the lowest two electronic states of the HNC1 molecule (X̃ ²A″ and à ²A'), and the corresponding vibrational frequencies and rotational energies have been computed using the non-rigid bender Hamiltonian. For the ground state the vibrational frequencies obtained are ν₁ = 2942 cm⁻¹, ν₂ = 1232 cm⁻¹, and ν₃ = 549 cm⁻¹, while the corresponding values for the first excited state are 3524,947 and 836 cm⁻¹ respectively. We calculate T_c(à ²A') 16200 cm⁻¹, T_o(òA') = 16400 cm⁻¹, and the Franck-Condon maximum, Ã(0,3,1)-X̃(0,0.0), is calculate at 19200 cm⁻¹(5200 Å).     

Observation of photodissociation of S2Cl2 and resonance Raman and fluorescence spectra of S2Cl under 514.5 nm radiation

The laser Raman spectrum of S₂Cl₂ varies with the sample temperature and/or the laser power. The Raman signals of S₂Cl₂ decreases as the sample molecules within the laser beam are dissociated by absorbing 514.5 nm photons. Above 540 K and 2 W of laser power, new resonance Raman and fluorescence bands appear. These bands were all assigned to S₂Cl. The fluorescence bands could be classified into two transition systems. Only one of them had the ground electronic state X̃ as its lower state. For the other, the low lying first excited state à was suspected. The fundamental frequencies suggested for the three vibrational modes were 664, 196 and 450 cm⁻¹ for the X̃ state and 630, 249 and 554 cm⁻¹ for the à state respectively.     

Ion dip spectroscopy as a means of probing fast predissociating species: NH3 Ã1A'2 state

Using NH₃ ùA''₂ state as an illustrative example, we have succeeded in developing ion dip spectroscopy into an effective means of probing fast predissociating species that should be applicable to a large number of other interesting cases. The ion dip process we studied was a folded optical-optical double resonance: NH₃ X̃ + 3hv₁ → NH₃ C̃ hv₂→NH₃ à with ion detection. Based on our “composite” rotationally resolved spectra, band origins of the C'-A transition and rotational constants for the NH₃ à v''₂ = 0, 1, 2 vibronic states are obtained, the latter with ±0.06 cm⁻¹ accuracy. A lifetime for NH₃ à v''₂ = 1 level as short as 120±10 fs (46±5 cm⁻¹ bandwidth) has been measured.     

The visible spectrum of jet-cooled CF3NO

The electronic origin of the à A″ ← X̃ A′ transition of trifluoronitrosomethane (CF₃NO) has been reassigned to the very weak feature near 717.9 nm in the fluorescence excitation spectrum of the jet-cooled molecule. The prominent torsional progression has been reinpreted and the height of the threefold torsional barier in the Ã( n,π^* ) state has been revised to 601.5 ± 10 cm⁻¹.     

Electronic spectra and predissociation of jet-cooled CH3O and CH3O-Ar in the Ã2A1 state

The òA₁-X̃²E electronic spectrum of jet-cooled methoxy radical has been examined by the LIF technique. Two newly discovered vibrational bands of 2930 and 1390 cm⁻¹ are assigned to ν₁ (totally symmetric C-H stretching) and a degenerate ν₅ mode, respectively. The predissociation, CH₃O → CH₃ + O in the òA₁ state is newly elucidated and the threshold energy is deter- mined as 5100 cm⁻¹ from the potential minimum. For the CH₃O-Ar complex, the threshold energy is reduced by about 100 cm⁻¹.     

Flash photolysis of SiBr4: the UV spectrum of SiBr2

In the flash photolysis of SiBr₄ both the absorption and the emission spectra corresponding to the B̃²Σ−X̃²Π transition of SiBr have been observed. A broad, structureless absorption band has also been detected in the 340–400 nm region which could be assigned to the hitherto unreported à ¹B₁−x̃ ¹A₁ transition of SiBr₂. The decay of both absorption spectra followed first-order kinetics yielding the pseudo-first-order rate constants: k(SiBr)=2.6 × 10⁴s⁻¹ and k(SiBr₂) = 8.9 × 10³⁻¹. Assuming that the principal reactions consuming these intermediates are SiBr+SiBr₄→Si₂Br₅ and SiBr₂+SiBr₄→ Si₂Br₆, the second-order rate constants have the values k(SiBr)= 9.7×10⁹ M⁻¹s⁻¹ and k(SiBr₂)= 3.3×10⁸M⁻¹s⁻¹.     

Spectroscopic characterization of organic cations: emission and laser-excitation spectra of rotationally cooled CH3CCX+, X  Cl,Br

The gas phase emission and laser-excitation spectra of the òE ↔ X̃²E transition of rotationally and vibrationally cooled 1-chloro- and 1-bromopropyne cations, and their fully deuterated analogues, have been obtained. The emission was excited by electron impact on a seeded helium supersonic free jet and the fluorescence by laser-excitation of cations formed by Penning ionization and collisional relaxation of liquid-nitrogen temperature. Comparison of the two sets of data locates the band origins and enables the prominent spectral features to be vibrationally assigned. The vibrational frequencies of many of the fundamentals could be inferred for these cations in their X̃²E and òE states.     

Copper trimer: a revised assignment of the upper state of the 5397 Å system

In light of recent fluorescence spectra obtained for the 5397 Å system of Cu₃, this system is reassigned as à ²A₁-X̃ ²E'. This assignment explains the observed bands without the need to invoke vibronic or Coriolis perturbations. It also accounts for the anomalous pattern of linewidths observed for the higher vibronic levels of the à ²A₁ state as unresolved splittings of the multiply excited doubly degenerate bending mode. The implications for the analysis of the ground state potential energy surfaces are discussed.     

Photoionization electronic spectroscopy of AlNa

Two electronic transitions are observed for a new heteronuclear dimer, AlNa, using two-color two-photon ionization spectrocopy (2R2PI). The combination of the analysis of vibronic structure and ab initio calculation enables us to assign the two transitions as D ¹Π←X ¹Σ⁺ and C ¹Σ⁺←X ¹Σ⁺; the origins of the D and C states are located at 16427 and 15422 cm⁻¹, and the vibrational frequencies being 151 cm⁻¹ in the D state, 115 cm⁻¹ in the C state, and 186 cm⁻¹ in the X state, respectively.     

Intramolecular vibrational state mixing in glyoxal by stimulated emission pumping spectroscopy

Stimulated emission pumping spectra of glyoxal in a supersonic jet were recorded up to 7000 cm⁻¹ above the ground vibrational state employing X̃(¹A_g) ← Ã(¹A_u)2¹₀ vibronic transitions. At low energies, only isolated states were observed. However, at high energies, clumps of eigenstates were observed, which suggests extensive vibrational state mixing with dense bath states.     

Time-resolved tunable diode laser detection of products of CF2HCl IRMPD: A linestrength measurement for CF2

Tunable diode laser transient detection of CF₂ C₂F₄, and HCl following infrared multiphoton dissociation (IRMPD) of CF₂HCl has been achieved. Quantification of the HCl and C₂F₄ leads to the calculation of an infrared absorption linestrength and the ν₁ bandstrength for CF₂ (X̃¹A₁). In addition, the rate coefficient for recombination of CF₂ was found to be (1.4± 0.4) × 10¹⁰ cm³ mol⁻¹ s⁻¹.     

Dichlorosilylene: rate constant for reaction with oxygen

Using the recently detected intense UV absorption spectrum of SiCl₂(X̃¹A₁) the absolute rate constant has been measured for its reaction with oxygen. The value of the bimolecular rate constant, k(SiCl₂ + O₂), is equal to (3.4 ± 0.7) × 10⁹ M⁻¹ s⁻¹ at 25° C.     

Collisional quenching of NO(A,v' =0) by various gases

The rate coefficients for the quenching of NO(A ²Σ, v' =0) by NO, N₂O, H₂O, O₂, SF₆ and CO₂ were measured to be (in units of 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹): 2.67±0.41, 3.79±0.49, 7.8±1.0, 1.46±0.25, 3.82±0.49, and 4.30±0.26, respectively. Upper limits for the quenching rate constants by N₃, Ar, Ne, H₂, and CF₄ were also measured. All experiments were carried out by monitoring the temporal profiles of A ²Σ, v' =O→X²Π, v″=3 fluorescence after 226 nm pulsed laser excitation of NO(X²Π) to NO(A²Σ,v'=0).     

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.     

Theoretical investigation of the character of the electronic states of H2O along a linear dissociation path leading to OH + H

A series of MRD CI calculations is presented for the linear asymmetric dissociation of singlet states of the water molecule. The present calculations complement previous work on H₂O, providing further information on conical intersections in the higher excited states and on the nature and dissociation limits of the third ¹A' state in C_s symmetry, which is shown to correlate with the ion-pair limit (OH⁻ +H⁺). At large distances the ion-pair state is no longer the third ¹A' species as other states, corresponding to the OH(X ²Π) + H(2s,2p) dissociation limit, are then lower in energy.     

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.     

C2O(Ã 3Πi↔-~X3Σ−: laser induced excitation and fluorescence spectra

Laser induced fluorescence of C₂O is observed following the 266 nm laser photodissociation Of C₃O₂. Excitation spectra of C₂O(óΠ_i?-~X³Σ^? are consistent with previous absorption studies of C₂O. A number of new transitions are identified and assigned. Fluorescence spectra have been recorded following single vibrational level laser excitation. Bands are assigned to ground state vibrational progressions. Values of 1967 and 1063 cm^?1 are found for υ₁″ and υ₃″ stretching vibrations in the X?³Σ ^? state. A subband structure in the fluorescence spectrum is observed and discussed.     

The electronic absorption spectrum of ICCCCI+ in a Ne matrix

The optical absorption spectrum of diiododiacetylene cation was obtained in a 4.5 K Ne matrix in the wavelength range 2800–9000 Å. The òΠ_3/2,u  X? ²Π_3/2,g transition was observed and vibrationally analysed.     

Detection of nitrogen rotational distributions by resonant 2 + 2 multiphoton ionization through the a1Πg state

Characterization of laser 2 + 2 multiphoton ionization of nitrogen to obtain rotational state distributions has been investigated via the resonant two-photon transition a ¹Π_g(ν = 1) ← X ¹Σ_g(ν = 0). For room-temperature nitrogen, the spectral intensities and state distribution are directly related and give rotational temperatures of 290 ± 20 K. For power densities of 3 GW/cm², the ionization probability is 1 × 10⁻⁵ per N₂ molecule per average rotational state.