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.

K-Rotational Relaxation of S1 H2CO (v4 = 1, JKaKc = 101) via Dispersed Fluorescence Spectroscopy

“…The rate constants are observed to have a modest pressure dependence. This is most likely a consequence of the unresolved J -relaxation that occurs concurrently with K -relaxation, coupled with J - and K -dependent fluorescence lifetimes. − However, the ratio of the state-to-state rate constants k 02 to the total depopulation rate constants k 0 is relatively constant and consistent between both models. The data in Table imply that approximately 53% of the observed K -depopulation occurs as a Δ K a ‘ = 2 step with rate constant k 02 and approximately 5% of the observed K -depopulation occurs as a Δ K a ‘ = 4 step with rate constant k 04 .…”

“…Hence, J -relaxation within a spectroscopic branch can occur concurrently with K -relaxation from that branch. Since the lifetimes of S 1 4 1 H 2 CO rotational states are known to depend on J , − variation of the population distribution among different J states in the same spectroscopic branch with pressure may account for the observed curvature in the Stern−Volmer plot.

6 Stern−Volmer plot of the experimental decay rates of the K a ‘ = 0 parent state. A second-order polynomial is fit to the data over (a) the entire pressure range studied in this work and (b) the pressure range 50−350 mTorr.

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“…Similar to the decoupled model, it is assumed that J -relaxation populates between one and five J‘ states, and the corresponding average scaling factor is used in eq 79. The coupled model assumes equal fluorescence decay rates for the K a ‘ = 0 and K a ‘ = 2 manifolds in eq 69, whereas S 1 H 2 CO lifetimes are known to depend on K a . − However, J -relaxation tends to reduce the effect of lifetime fluctuations. The values of k 02 obtained at each pressure are reported in Table .…”

Investigation of droplet ignition under microgravity conditions using laser-based techniques—an overview

“…Due to rotational and vibrational equilibration on a time scale shorter than the uorescence lifetime, the 4 0 niveau in S 1 is occupied. The vibrational relaxation between 4 1 and 4 0 is 10 3 times more e cient than collision induced electronic quenching by bu er gas molecules [22][23][24]. The population of the 4 1 and 4 0 levels corresponds to Boltzmann's law.…”

Spektroskopische Erfassung der Gastemperatur im Brennraum von Ottomotoren

“…Die Indizes M stehen für Formaldehyd und P i für das Puffergas i. Shibuya et al [39], [40] haben im Rahmen der Messgenauigkeit folgende Werte ermittelt:…”