Collision-free Radiative Lifetime Research Articles

Laser-induced fluorescence from selected excited states of the CS 2 molecule

Gas-phase samples of CS 2 exhibit a richly structured absorption spectrum in the near-ultraviolet region extending from roughly 2900 to 3800 Å. In this context, the fluorescence of the CS 2 molecule was excited by a dye laser pumped by a Nd-YAG laser in the spectral interval 2800–3380 Å. From the analysis of the fluorescence emission decay of the CS 2 molecule, two different processes were observed as the excitation wavelength was tuned from 2800 to 3380 Å. Collision-free radiative lifetimes were determined and values for quenching rate constants in the range k Q ≈ (0.8 ± 0.3) × 10 −10 −(3.6 ± 0.7) × 10 −10 cm 3molecule −1s −1 in the spectral region studied were obtained.

Laser-induced fluorescence of the CH2CFO radical

A new laser-induced fluorescence spectrum has been observed in the region of 307–335 nm. Since this spectrum is observed when reacting oxygen atoms with CH2CHF, or CH2CF2, or CH2CFCl and also by photolysis of CH3CFO, the fluorescing molecule is the CH2CFO (fluoroformyl methyl) radical. From an analysis of the laser-induced single vibronic level fluorescence, some of the vibrational frequencies can be assigned for the ground electronic state ν3=1724 cm−1 (C–O stretch), ν5=1211 cm−1 (C–F stretch), ν6=906 cm−1 (CH2 rock), ν7=847 cm−1 (C–C stretch), ν8=584 cm−1 (FCO bend), and ν9=416 cm−1 (CCO bend), for the excited state ν3=1790, ν5=1253, ν6=911, ν7=874, ν8=537, and ν9=421 cm−1. Ab initio calculations on the CH2CFO radical give a planar geometry with vibrational frequencies that are consistent with the observed fundamental frequencies. The vibrational frequencies show that the structure of the ground state is closer to fluoroformyl methyl (⋅CH2CFO) rather than a vinoxy-type (CH2=CFO⋅) radical. The collision-free radiative lifetimes of the excited state are 49–81 ns depending on excitation energy and vibrational modes. Strong predissociation is observed above v=1, especially in the ν3′ mode.

The A(0+)−X(0+) transition of BiBr: radiative lifetimes of the A state

Abstract Laser induced fluorescence from resolved rovibrational levels ( ν ′, J ′) of the electronically excited A(0 + ) state of BiBr has been observed. Fluorescence lifetimes for several rovibrational levels ( ν ′, J ′) of the BiBr A(0 + ) state in the ν ′ = 4–9 interval have been measured, and a study of their variation with pressure has been made. No significant variations of radiative lifetimes from specific vibrational levels ν ′ ( ν ′ = 0–9) of the BiBr A excited state has been observed, and no dependence on the rotational energy, within a vibrational level, was detected for the measured lifetimes. A collision-free radiative lifetime of τ 0 ≈ 0.52 μ s is proposed for the BiBr A(0 + ) state and values for quenching rate constants in the order of k Q ≈ (1.0 ± 0.3) × 10 −11 cm 3 molecule −1 s −1 in the spectral range studied were obtained. From calculated Franck-Condon factors for the ν ′− ν ′' transitions of the A-X system and from our radiative lifetimes, electronic transition moments have been calculated.

Radiative lifetimes of CH2(b 1B1)

Radiative lifetimes are reported for CH2(b 1B1) in a wide range of rotational states in the (0,14,0), (0,15,0), (0,16,0), and (0,17,0) vibrational levels. Laser photolysis of ketene in a supersonic-jet expansion produced CH2(a 1A1) which was excited to single rovibrational levels of the (b 1B1) state by a second laser. Analysis of the temporal evolution of the fluorescence (b 1B1)–(a 1A1) yielded collision-free radiative lifetimes for the (b 1B1) state. The measured lifetimes range from about 4 to 10 μs and decrease with increasing vibrational energy. For the (0,14,0) overtone, the lifetimes increase slightly as a function of J (Ka=0). However, the lifetimes of rotational levels with Ka=0 in the (0,16,0) vibrational state are found to be independent of the rotational state. Calculations of the vibronic lifetimes show that the considerable lifetime lengthening when Ka≥1 is due to Renner–Teller coupling to the ã 1A1 state. The random lifetime fluctuation observed in some vibronic bands is probably due to spin–orbit coupling to the X̃ 3B1 state, e.g., 413 and 414 of (0,15,0). Here the radiative lifetime of the singlet component is shorter than that of the corresponding triplet component.

Laser-induced fluorescence excitation spectrum and radiative lifetime of Cl2 +

Laser-induced fluorescence from the Cl2+ molecular ion has been observed for the first time and the excitation spectrum of the A(2Πu)–X(2Πg) transition recorded at rotational resolution. Although the rotational structure is regular, the vibrational levels of the upper state are heavily perturbed by an unknown state which has not been observed in emission experiments. A number of new bands are detected in this work which are assigned to transitions involving this previously unobserved perturbing state. The ions are generated in a supersonic free jet using a pulsed discharge technique and are observed in the collision-free region of the expansion. Substantial variation in the collision-free radiative lifetimes are observed among the vibrational bands of both the A(2Πu) state and in the perturbing state. This is explained by considering the substantial mixing that occurs between these two states.

High-resolution coherent VUV spectroscopy of NO[C 2Π(1)∼B 2Π(10), B 2Π(11)] and CO[B 1Σ(0)

The outputs of two single longitudinal mode dye lasers have been combined by the processes of four-wave sum and difference frequency mixing in Hg vapour to produce tunable, coherent VUV radiation in the ranges 110–116 nm and 178–195 nm, respectively. The dye laser pulses of 500 MHz bandwidth (fwhm) produced a VUV pulse of bandwidth 3 GHz (0.1 cm −1, fwhm).The bandwidth was tested by recording UF spectra of NO[B 2Π←X 2Π(10, 0) and (11, 0)] and CO[B 1Σ +←X 1Σ +(0, 0)]. This VUV light was also used to excite single rovibrational levels of NO or CO in a supersonic jet and the resulting radiative decay was monitored by time-resolved fluorescence detection. Collision-free radiative lifetimes were measured for the first time for several low J rotational levels of B 2Π(10) and B 2Π(11) of NO. The B 2Π(10) level showed a variation in lifetime of 60.6±2.0 to 39.3±5.0 ns from J = 1 2 to J = 11 2 while B 2Π(11) showed no variation in its lifetime of 14.0±0.4 ns. Linewidth measurements for the C 2Π(1) level infer a lower limit of 0.03 ns for its lifetime. Lifetimes were also measured for B 1Σ +(0) of CO whose radiative decay revealed severe radiation trapping effects at pressures above 10 −5 Torr.

Vacuum ultraviolet laser spectroscopy: Radiative lifetimes of interacting 2Δ states of NO

Interactions between the B′ 2Δ non-Rydberg state and nearby 2Δ Rydberg states of NO have been studied by vacuum ultraviolet laser-induced fluorescence spectroscopy. By using tunable, coherent radiation in the 60 000–70 000 cm−1 range, NO molecules in a supersonic jet were excited and their radiative decay was monitored by time-resolved fluorescence detection. In this way accurate, collision-free radiative lifetimes were measured for several levels of the B′ 2Δ, F 2Δ, N 2Δ, and D 2Σ+ excited states of NO. Using these lifetimes the oscillator strengths for the B′–X and F–X transitions were calculated. The radiative lifetimes for the perturbed states were quite different from the unperturbed lifetimes, and these changes are discussed in terms of the known interactions between states in this spectral region. For the unperturbed vibrational levels of the B′ 2Δ state, radiative lifetimes between 129–139 ns were measured, while for the F 2Δ state, a radiative lifetime of 30 ns was measured. For the perturbed bands of the F2Δ and B′ 2Δ states, the radiative lifetimes were accurately described by a two-state homogeneous perturbation model, but for the B′(v=7)∼N(v=0) mixed states, the lifetimes were dramatically different from those expected on the basis of two interacting states. This is caused by a third-interacting state whose identity is not known. The B′(v=8,9) states, while having unperturbed line positions, have shorter radiative lifetimes, indicating the onset of predissociation in those levels.

Laser-induced fluorescence of the SiCl 2 radical

Dichlorosilylene, SiCl 2, in the gas phase has been detected by laser-induced fluorescence of the à 1B 1-X̃ 1A 1 transition in the 309–333 nm region. The wavenumber of the 0 0 0 transition is determined to be 30003.6 ± 0.5 cm −1 (333.20 nm). The fundamental vibrational frequencies are assigned to be v″ 1 = 526.5 cm −1, v″ 2 = 201.2 cm −1 (X̃ 1A 1), v' 1 = 435 ± 5 cm −1, and v' 2 = 149.9 ± 0.5 cm −1 (à 1B 1). The collision-free radiative lifetime of the 1B 1 state is 77 ± 3 ns at v' 2 =7 (321.949 nm).

Dynamics of 14N16O and 15N18O excited with an ArF-exciplex laser at 193 nm

The dynamics of the B 2∏r(ν' = 7) state of 14N16O and the heavy-heavy molecular isotope 15N18O was studied in a laser induced fluorescence bulb system at room temperature. The NO was excited at 193 nm with an exciplex laser. The fluorescence was observed spectrally and time resolved. The NO pressure was varied in the range 10 ˇ- p(15N 18O)/Pa ˇ- 140. Quenching gases He, Ar, Xe, and N2 were added from 13 Pa up to about 90·0 kPa. For 15N18O the collision-free radiative lifetime was determined to be τo(NO) = 0·38 ± 0·05 μs and the self-quenching rate was found to be k Q(NO) = (1·7 ± 0·2) × 1014 cm3 mol-1 s-1. A significant difference with the 14N16O isotope was not found. Quenching rates for Ar and N2 are determined to be k Q(Ar) = (4·3 ± 1·5) × 1013 cm3 mol-1 s-1 and k Q(N2) = (7·9 + 2·0) × 1013 cm3 mol-1 s-1. The quenching mechanism is discussed.

Experimental and theoretical study of the Ca I 4s3d1D-4s21S and 4s4p3P1-4s21S forbidden transitions

A time-of-flight technique for the determination of forbidden (ms time range) collision-free radiative lifetimes is presented. In this method, the time-of-flight spectrum of metastable atoms generated from a molecular beam source is compared with that for a non-radiating (ground or metastable) state, and the lifetime is derived from a semi-logarithmic plot of the ratio of intensities against flight time. The lifetimes of the calcium 4s4p 3P1 and 4d3d 1D states are determined to be 0.50+or-0.04 (2 sigma ) and 2.3+or-0.5 (2 sigma ) ms. The former result is in reasonable agreement with earlier determinations of this lifetime. One radiative decay pathway for the 1D state is direct electric quadrupole radiation to the ground state. The transition rate for this process has been determined by a MCSCF/CI calculation.

Critically evaluated rate constants for gaseous reactions of several electronically excited species

An extensive evaluation is presented of the available gas phase chemical kinetic rate constants for the interactions of the low lying electronic states of several atoms and molecules with numerous collision partners. These include the following excited states: C(21D2,21S0), N(22D3/2,5/2,22P1/2,3/2), P(32D3/2,5/2,32P1/2,3/2,), S(31D2,31S0). Se(43P0,41D2,41S0), Te(53P1,0,51D2,51S0), CO(a3Π,a′3Σ+,d3Δ,e3Σ−,A1Π), CS(a3Π,A1Π), OH(A2Σ+), OD(A2Σ+), O2(c1Σu−,C3Δu,A3Σu+,B3 Σu−), and S2(a1Δg,b1Σg+,A3Σu+, B3Σu−). Wherever possible, recommended values are suggested. Much of the data refers only to room temperature. To facilitate the evaluation, collision-free radiative lifetimes often have been required. These also have been evaluated and are presented. The mechanisms of the interactions and the various potential kinetic channels are discussed. These include such processes as chemical reactions, electronic quenching to the ground electronic state, electronic cross relaxation to an adjacent excited state, and for molecules, vibrational and rotational relaxation processes within the excited state. A complete coverage of the literature published prior to 1978 has been attempted.