J. H. Clark

HCO production, vibrational relaxation, chemical kinetics, and spectroscopy following laser photolysis of formaldehyde

Formaldehyde vapor was photolyzed with a tunable pulsed uv laser. Flash kinetic absorption spectra of the HCO produced were recorded by intracavity dye laser spectroscopy with a time resolution of 1 μs. The energy threshold for radical production was confirmed to be at 86±1 kcal/mole. Photolysis at 294.1 nm produced HCO in its ground vibronic state (∼2/3) and with one quantum of vibrational excitation in either the bending (∼1/3) or CO stretching (10−1–10−2) vibrations. Observation of the CO stretching hot band absorptions allowed that frequency to be determined as 1868.4±1 cm−1. Quantitative, state‐resolved measurements of concentration vs time were made in pure H2CO and in mixtures with O2, NO, or Ar. The vibrational relaxation rate for the bending vibration of HCO in collisions with H2CO was (4.3±1) ×10−12 cm3 molecule−1 s−1. Reaction rates for HCO+NO→HNO+CO and HCO+O2→HO2+CO were measured as (1.4±0.2) ×10−11 and (4.0±0.8) ×10−12 cm3 molecule−1 s−1, respectively. Approximate rates were determined for t...

High-pressure studies of rotational reorientation dynamics: The role of dielectric friction

Picosecond, time‐resolved, fluorescence depolarization spectroscopy is used to measure the rotational reorientation times of rhodamine 6G (R6G) and p‐terphenyl (PTP) as a function of solvent viscosity. The viscosity is varied either by changing the solvent or by changing the pressure in a single solvent. The differences between the two molecules, PTP and R6G, provide a means of evaluating the role of solute structure and solute–solvent interactions on the dynamics of rotational reorientation. The rotational behavior of PTP is well described by simple hydrodynamic models as embodied by the Stokes–Einstein–Debye equation. In contrast, the rotational reorientation dynamics of the charged molecule R6G are not well described by these models. It is demonstrated that dielectric friction plays an important role in governing the rotational motion of charged molecules in polar solvents. When the solvent dielectric properties are varied, the dielectric friction model accurately predicts the observed experimental tre...

The photochemistry of formaldehyde: Absolute quantum yields, radical reactions, and NO reactions

Absolute quantum yields have been measured for the gas phase photolysis of H2CO and D2CO as a function of excitation energy. Over the spectral region examined, 290 nm ?λ?360 nm, formaldehyde can dissociate into either molecular or free radical products: H2CO+hν→H2+CO or H2CO+hν→H+HCO. For H2CO the total photochemical quantum yield was Φtotal=1.0±0.1 for λ?326 nm and 0.68 at λ=353 nm. For D2CO, Φtotal=1.0±0.1 for λ?331 nm and drops to zero near the band origin. The mechanism of formyl radical recombination in pure formaldehyde was shown to be 2HCO→H2CO+CO. Addition of NO to the photolysis mixture allowed determination of the radical/molecule branching ratio by introducing the scavenging reactions: HCO+NO→HNO+CO, and H+NO+M→HNO+M. The threshold for production of radicals was found to be between 339 nm and 330 nm for H2CO, and between 326 nm and 320 nm for D2CO. Radical quantum yields increase steeply at the threshold and remain constant near 0.4 at higher energies.

Carbon isotope separation by tunable-laser predissociation of formaldehyde

Separation of carbon 1213 isotopes has been achieved by selective predissociation of formaldehyde using a frequency-double dye laser. The enrichment factor for pure formaldehyde samples is less than expected from spectroscopic data but is greatly improved by the addition of NO. Starting from a 1:10 mixture of 12CH2O:13CH2O an eighty-fold enrichment in 12CO has been obtained.

Direct observation of the excited-state proton transfer and decay kinetics of internally hydrogen-bonded photostabilizers in copolymer films

The excited-state dynamics of a 2-hydroxyphenylbenzotriazole (HPB) photostabilizer copolymerized with polystyrene are reported. HPB fluorescence from these copolymer films is observed at ≈630 nm, characteristic of the proton-transferred excited state of HPB, and it has a risetime of <10 ps and a decay time of 28±4 ps at room temperature. Measurement of the relative fluorescence quantum yield as a function of temperature gives the activation energy for non-radiative decay of this state to be E/hc = 259±25 cm−1.