Non-linear pressure dependence of A-state fluorescence lifetime of formaldehyde

Abstract

Fluorescence lifetimes of formaldehyde excited at 352 nm ( $$\tilde A^1$$ A2 − $$\tilde X^1$$ A1 401 band) were measured as a function of bath gas pressure. He, N2, O2, CO2 and HCHO were investigated for the bath gas and the temperature dependence between 298 and 500 K for N2 and O2 bath gases was also examined. It was found that the non-linear pressure dependence of the lifetime τ is successfully reproduced by the model formula $$ \tau^{-1} = k_{\rm f} + (k_{\rm q} + k_{\rm a})[{\rm M}] - \frac{k_{\rm a} k_{\rm b}}{k_{\rm b} [{\rm M}] + k_{\rm p}}[{\rm M}]^2,$$ where [M] is the concentration of a bath gas and kf, kq, ka, kb and kp are the constants determined for each bath gas. This model assumes that the optically excited formaldehyde undergoes a reversible collision transfer to a state of higher spontaneous decay rate along with direct collisional and spontaneous deactivation pathways. It was confirmed that a lifetime in a bath gas mixture can be reproduced by this formula with the constants individually obtained as linear combinations of each bath gas contribution. The temperature dependence is expressed by assigning activation energies for the constants in the formula.