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

Abstract

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′)〉 = 0.033 and the average value of radiative lifetimes is τrH = 3.3±1.2 μsec for the 41 level.