Paul W. Fairchild

Relative quantum yields of O(1D) in Ozone Photolysis in the region between 250 and 300 nm

Abstract Relative quantum yields of O( 1 D 2 ) formation, φ rel , for ozone photolysis in theregion of 250–300 nm have been determined in the gas phase at 23°C. The NO * 2 chemiluminescence resulting from the photoexcitation of an O 3 /N 2 O mixture was usedin monitoring the O( 1 D 2 ) formation. The results show that the value of Φ rel remains constant throughout the photolysis wavelength range studied at 2–5 nm intervals.

A photochemical study of rotational state dependence by laser excitation of formaldehyde (? 1A2). I. Coriolis and singlet–triplet perturbation

A study of rotational dependence of radiationless transition rate and radical yield in the photodecomposition of formaldehyde, H2CO(? 1S2), was made with laser‐induced fluorescence excitation spectroscopy and chemiluminescence excitation spectroscopy [Lewis, Tang and Lee, J. Chem. Phys., 65, 2910 (1976)]. The effect of perturbations on some rovibronic levels excited by the 220410 and 230410 transitions is examined.

Single rotational level fluorescence quantum yields, radiative lifetimes, and nonradiative decay rates of S1 D2CO and H2CO(Ã 1A2, 41): Rotational dependence

A large number of rotational levels in the 41 (v4′ = 1) manifold of S1 formaldehyde were excited in a multipath absorption/fluorescence cell using a pulsed dye laser. Fluorescence decay times (τfD) of D2CO were measured for a number of rotational levels at 10.8 mTorr so that the apparatus could be calibrated for the measurement of fluorescence quantum yields (ΦfH) of many rotational levels of H2CO at varying pressures (1–120 mTorr). For 10.8 mTorr D2CO, the average values of τfD and ΦfD were 4.8±0.3 μsec and 0.66±0.07, respectively. The zero pressure values of ΦfH for H2CO varied randomly from 0.0063 for J′ = 13, K′ = 7 (Erot = 570.9 cm−1) to 0.32 for J′ = 2, K′ = 2 (Erot = 37.1 cm−1), due to a random variation of the nonradiative decay rates. The J′‐population averaged value of ΦfH in a given K′ manifold 〈ΦfH(J′)〉K′ shows a trend to decrease with the increase in the K′ quantum number for K′ = 2–6, but becomes nearly constant for K′ = 6–10. The (J′, K′)‐population averaged value of ΦfH is 〈ΦfH(J′, K′)〉 = ...

Pressure dependence of fluorescence quantum yields and collision‐induced rotational relaxation of single rotational levels of H2CO(Ã 1A2, 41)

Pressure dependent values of SRL fluorescence quantum yields Φf(J′, K′) in the 1–120 mTorr range are reported for 75 rotational levels (E′rot = 37–1127 cm−1) of S1 H2CO (41). High lying rotational levels tend to show an increase in Φf with increasing pressure of S0 H2CO, whereas low lying rotational levels tend to show a decrease. For some levels, collision‐induced ΔJ′ transitions occur 5–10 times faster than the gas kinetic collision rate but probably with a much slower rate for ΔK′ transitions. A ΔK′‐constrained, collision‐induced rotational relaxation model involving mainly ΔJ′ = ±1 transitions is proposed to explain the observed pressure dependence of Φf(J′, K′).

A fluorescence monitor method for measuring effective absorption coefficients of molecular rovibronic transitions using tunable dye laser excitation: The case of absorber linewidth narrower than the laser linewidth applied to H2CO

A technique for measuring ’’effective’’ absorption coefficients is described. It circumvents deviations from Beer’s law caused when the excitation source bandwidth is larger than the absorber bandwidth. The technique employs a fluorescence cell placed after an absorption cell to selectively monitor absorption in the center region of the source line. Model calculations relating the fluorescence intensity to source and absorber line shapes indicate that this method should yield linear Beer’s law plots for moderate values of k0Nl and α, where α is the ratio of the source bandwidth to the absorber bandwidth. This technique has been applied to a number of single rotational levels in the 410 transition of the H2CO ? 1A2←? 1A1 system using pulsed, tunable dye laser excitation. The effective absorption coefficients determined experimentally have been compared to the theoretically calculated absorption coefficients.