K.H. Welge

Photodissociation dynamics of H2S at 121.6 nm and a determination of the potential energy function of SH(A 2Σ

A new and improved version of the technique of H atom photofragment translational spectroscopy has been applied to a study of H2S photodissociation at 121.6 nm. The primary fragmentation pathways leading to H+SH(A) fragments and H+H+S(1D) atoms are observed to dominate the product yield; the yield of H atoms formed in conjunction with ground state SH(X) fragments is undetectably small. The majority of the SH(A) fragments are formed in their v=0 level with a rotational state population distribution that spans all possible bound and quasibound rotational levels. The experimental determination of the energies of these hitherto unobserved high rotational states has enabled a refinement of the SH(A) potential energy function, an improved estimate of the SH(A) well depth (9280±600 cm−1), and thus of the SH(X) ground state bond dissociation energy D00 (S–H)=3.71±0.07 eV. All aspects of the observed energy disposal in the title photodissociation process may be understood, qualitatively, if it is assumed that (i) ...

Primary product channels in the photodissociation of methane at 121.6 nm

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) at 121.6 nm. Contrary to the previous consensus view, we find simple C–H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH3 fragments are formed with very high levels of internal excitation: Some (∼25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(A 1T2) molecules undergo rapid internal conversion (promoted by the Jahn–Teller distortion of this excit...

Laser induced fluorescence from NH2(2A1). State selected radiative lifetimes and collisional de‐excitation rates

Time resolved fluorescence from the first excited (2A1) state of NH2 has been observed following excitation of the radical in its ground state by means of a pulsed tunable dye laser. Specific rotational levels within a number of vibronic states were populated, decay rates measured as a function of total pressure for a variety of added gases, and zero pressure lifetimes and collisional de‐excitation rates evaluated. Measured zero pressure lifetimes are good approximations to the vibrational state radiative lifetimes, typically 10 μsec for the (0, 9, 0) state. Collisional de‐excitation rate constants were measured as 1.0×10−9 cm3 molecule−1⋅sec−1 for NH3, independent of vibronic state, and for the Σ (0, 9, 0) level were found for other gases in the ratio NH3:CO:H2:N2:CH4:Ar:He=1.0:0.47:0.46:0. 40:0.30:0.152:0.145. Using excitation by a tunable cw dye laser, steady state spectra of NH2 have been obtained and collisional energy transfer observed within the (2A1) excited electronic state of NH2. Transfer was o...

Two-photon excitation of NO(A2Σ+; ν′ = 0,1,2) and radiation lifetime and quenching measurements

Abstract The NO molecule has been excited with tunable pulsed dye laser by two-photon absorption into ν′ = 0,1,2 levels of the A 2 Σ + state at bandwidths down to Doppler width. The method has been used to measure the radiation lifetime and the collisional deexcitation of rotationally resolved levels in the A 2 Σ + state.

Laser Doppler spectroscopy of atomic hydrogen in the photodissociation of HI

Laser induced fluorescence Doppler spectroscopy has been applied for the first time to atomic hydrogen using tunable VUV light at the Lyman-α line. The dissociation of HI at 266 nm into H+I(P1/2) and H+I(P3/2) has been investigated. The recoil energy, angular distribution and branching ratio of the H atom have been measured, serving to test and study the feasibility and applicability of the technique.

Multiphoton ionization of methyl iodide and selective stepwise ionization of methyl radicals

The collisionless multiphoton ionization of methyliodide in the isotopic forms CH3I and CD3I has been investigated with pulsed laser light of two wavelengths: fixed‐frequency 266 nm of high intensity (∼1×107 W/cm2) and tunable around 215 nm of relatively lower intensity (∼2×106 W/cm2). The 215 nm was chosen because the methyl radicals CH3 and CD3 absorb there resonantly through the (0,0) (B 2A′1←X 2A′′2) band transition with the B state being very heavily predissociated. Objectives have been: (1) the ionization mechanism of CD3I to form ion fragments D+, CD+, CD2+, CD3I+, and I+, obtained with 266 nm alone, and 266 nm together with tunable 215 nm, (2) the possibility of influencing the MPI by selective excitation of an intermediate (CD3, CH3), and (3) detection and spectroscopy of the methyl radical by two‐photon ionization through the decaying B 2A′1 state as intermediate step. Mass spectra have been taken and measurements have been made with respect to the abundance of fragments, power dependence, se...

Laser fluorescence of NH2 and rate constant measurement of NH2 + NO

Abstract Laser induced fluorescence from the A 2 A 1 state of the NH 2 radical, obtained by different methods, has been observed with a tunable cw dye laser as excitation source. Using pulsed photolysis of NH 3 to produce NH 2 , the fluorescence technique has been employed in a first gas kinetic application to measure the rate constant of the aeronomically interesting NH 2 + NO reaction at 298 K. A value of (2.1 ± 0.2) × 10 -11 cm 3 molecule -1 s -1 has been obtained.

Resonant two photon ionization of H and D atoms

Abstract H and D atoms have been ionized with tunable VUV + UV laser light by two-photon absorption, resonantly through the 2 2 P 1 2 , 2 2 P 3 2 fine structure doublet, with total energies very close to the ionization potential. Experiments have been carried out in the ionization chamber of a quadrupole massfilter, in a crossed atomic-laser beam configuration with sub- Doppler resolution of ∼ 10 GHz. The sensitivity is ⪆ 2.5 x 10 5 atoms per cm 3 .

State selective step-wise photoionization of NO with mass spectroscopic ion detection

State and isotope selective two-step photoionization of NO with mass spectroscopic ion detection has been demonstrated and investigated. Using saturation condition the photoionization cross section for a single rotational level of the intermediate state, No(A2Σ+, ν′=0), has been measured: σi=(7.0±0.9) X 10−19 cm2. The charge transfer15NO++14NO→15NO+14NO+ has been observed and investigated, yielding a cross section of the order of 13×10−16 cm2, consistent with recent measurements at about 1 eV.

Laser induced fluorescence spectrum of singlet methylene. Spectroscopy of the a~1A1 state

Abstract CH 2 has been roduced by CO 2 -laser multiphoton dissociation of acetic anhydride. Selected levels of the b 1 B 1 state of CH 2 have been populated by dye laser induced excitation from the a state. The fluorescence spectrum of the b 1 B 1 -a 1 A 1 transition has been measured to determine vibrational and rotational constants of vibrationally excited CH 2 (a 1 A 1 ) up to the (2,0,0) and (0,3,0) levels.