Daniel C. Robie

Detection of Cl atoms and HCl molecules by resonantly enhanced multiphoton ionization

Abstract Resonantly enhanced multiphoton ionization has been used to detect Cl(3p 2P 3 2 o) and Cl*(3p 2P 1 2 o) via the pumping of two-photon-allowed transitions in the wavelength region of 230–245 nm. The precursors used to generate Cl atoms were CH2Cl2, CF2Cl2 and CCl4. Two-photon-allowed transitions of HCl and DCl molecules were also observed in this region, with a detection sensitivity of better than 1010 molecules/cm3.

The monoenergetic unimolecular reaction of expansion‐cooled NO2: NO product state distributions at excess energies 0–3000 cm−1

We report detailed vibrational, rotational, and electronic (V,R,E) distributions of nascent NO(X 2Π1/2,3/2) deriving from monoenergetic unimolecular reactions of expansion‐cooled NO2. Near UV excitation above dissociation threshold (25 130.6 cm−1) prepares molecular eigenstates which are admixtures of the optically active 1 2B2 state and the ground X 2A1 electronic state. The strong mixings among the vibronic states result in vibrational predissociation from states of predominantly ground state character, and the NO product state distributions (PSDs) are compared with the predictions of several statistical theories. The PSDs are combined with previously measured O(3PJ) distributions and unimolecular reaction rates, thereby providing a complete description of the decomposition of NO2 at these excess energies. All the rotational distributions show prominent fluctuations and structures, but tend on average to follow the statistical distributions predicted by phase space theory (PST). This behavior is observ...

The detection of bromine atoms by resonant multiphoton ionization

Abstract Br(4p 2 P 3/2 o ) and Br * (4p 2 P 1/2 o ) atoms have been detected by resonant (2 + 1) multiphoton ionization, using tunable laser radiation between 240 and 285 nm. The bromine atoms were generated by the photolysis of Br 2 . HBr, CH 3 Br and C 2 H 5 Br. A total of 21 atomic transitions were detected, and in addition several intense molecular bands of Br 2 , HBr and CH 3 Br were observed.

Fluctuations in the unimolecular decomposition of jet‐cooled NO2: Implications for overlapping resonances and the transition state

We report a detailed examination of fluctuations in the internal state distributions of the NO (2II) fragment formed in the monoenergetic unimolecular decomposition of jet‐cooled NO2, utilizing photofragment yield (PHOFRY) spectroscopy. The NO(v,J,Ω,Λ) PHOFRY spectra at E°<3000 cm−1 are highly structured, and we examine correlations among these spectra on the basis of the angular momentum, spin–orbit, parity, and vibrational labels of the monitored NO rovibrational levels. We find that levels of similar total angular momentum in the same vibrational manifold are more strongly correlated with specific resonances in the activated molecule, as well as levels of the same N in the two spin–orbit states of NO. The observed PHOFRY correlations, as well as those found in the NO(2ΠΩ; Ω=1/2,3/2) rotational state distributions, are interpreted in terms of projections of coherently excited overlapping molecular eigenstates onto the manifold of final states via levels of the transition state. The implications of the r...

Product state distributions in the photodissociation of expansion-cooled NO2 near the NO(X 2Π) ν=1 threshold

Abstract The photodissociation of NO2 near the NO ν=1 threshold was studied using a supersonic molecular beam of NO2 and multiphoton ionization detection of NO(X 2Π 1 2 , 3 2 ). The vibrational populations near the ν=1 threshold are nonstatistical as compared with the predictions of phase space theory (PST). The rotational distributions in both NO ν=0 and 1 show pronounced structures and fluctuations; however, their average is described fairly well by PST, suggesting that the decomposition of NO2 at λ ⩾ 370 nm can be viewed as vibrational predissociation on the mixed X 2A1/1 2B2 surface. Similar structured rotational distributions have been observed by Miller et al. in recent studies of the photodissociation of CO2 at 157 nm. The structures in the rotational distributions are interpreted as fluctuations inherent in the decomposition of an excited complex with many overlapping resonances (Ericson fluctuations).

Experimental probes of dissociative states: Fano profiles in the state‐specific photodissociation of FNO

We have observed strongly asymmetric features in the S1←S0 state‐specific photofragment yield spectrum of jet‐cooled FNO. We interpret these as Fano profiles, arising from interference between direct and indirect dissociation following excitation to a region near the top of a barrier on the S1 potential energy surface.

Evidence for tunneling in the reaction O(3P)+HD

Abstract The intramolecular kinetic isotope effect was determined for the title reaction by measuring laser-induced fluorescence from OH and OD in a discharge-flow apparatus. The OH/OD branching ratio was found to have a strong negative temperature dependence, showing that tunneling dominates the reaction at low temperatures. An apparent deviation from tunneling-corrected transition state theory calculations at low temperatures remains to be explained.

The intramolecular kinetic isotope effect for the reaction O(3P)+HD

The branching ratio for the reaction O(3P)+HD to produce OH and OD was measured over the temperature range 339–500 K using a discharge‐flow reactor. The OH and OD products were detected using laser‐induced fluorescence under steady‐state conditions. In order to determine the relative concentrations of OH and OD produced in this reaction, calibration runs were performed using the reactions of O(3P)+H2 and D2 to produce known relative amounts of the same species. Kinetic modeling showed that secondary reactions and the production of vibrationally excited OD did not affect the measured branching ratio. We found that the OH/OD ratio increased rapidly with decreasing temperature, in qualitative agreement with theory, showing that the reaction is dominated by tunneling below 400 K. At the lowest temperatures, the observed branching ratio is larger than predicted. The reason for this discrepancy is not known.

Generation of highly vibrationally excited H2 and detection by 2+1 resonantly enhanced multiphoton ionization

We report the first detection by optical means of highly vibrationally excited H2 X1Σ+g(vx=6–11). Vibrationally excited H2 was generated using a recently discovered hot‐wire effect in H2 gas, and was detected in 40 bands with 2+1 resonantly enhanced multiphoton ionization via the EF state (vEF=0–14). Rotational temperatures are in the range 200–650 K, well below that required for thermal excitation of the observed vibrational levels.

Bandwidth and tuning range of an ArF laser measured by 1 + 1 resonantly enhanced multiphoton ionization of NO

The tuning range and bandwidth of an ArF laser were measured using 1 + 1 resonantly enhanced multiphoton ionization of NO. Operated as an injection-seeded oscillator/amplifier combination, the tuning range was 51,560-51,810 cm(-1); operated with single pass amplification of the oscillator, the tuning range was 51,560-51,765 cm(-1). In both cases, the laser bandwidth, determined from the linewidth, was 0.21 +/- 0.06 cm(-1). Rotational lines in the beta(7,0), gamma(3,O), and epsilon(0,1) bands were observed including several previously unreported lines.