Abstract: The NH(A ~3Π)→X ~3∑~-)) fluorescence spectrum, of 331-343 nm, wavelength region was recorded when NH_3 was irradiated by the 193 nm ArF laser beam(Fig.1). The ratio of the intensity I_f of Q(0, 0) to that of Q(1, 1) suggests that 5% of population of NH(A ~3Π) is in the first vibrational state (v′=1). For the ro- tational lines of the P branch, a diagram of In I_f/(2N′+1) vs. Be N′(N′+1) is plotted in Fig.2, in which I_f corresponds to the relative intensities of the ro- tational lines of the P branch and N′ is the rotational quantum number of NH(A ~3Π, v′=0). NH(A ~3Π) is highly rotationally excited in the firt 12 levels with a Boltzmann temperature of T_R=7700 K.
A single-exponential decay of the NH(A ~3Π→X ~3Σ~-) emission was found and a lifetime of (470±40) ns was obtained. The Sterm-Volmer plots are shown in Fig.3. The quenching rate coefficients of NH(A ~3Π) by various gases were measured from the slopes as: k_(H_2)=(4.8±0.5)×10~(-11) cm~3 molec~(-1) s~(-1), k_(NO)=(7.3±1.0)×10~(-11) cm~3 molec~(-1) s~(-1), k_(CH_4)=(6.6±1.5)×10~(-11) cm~3 molec~(-1) s~(-1), and k_(O_2)=(2.9±0.2)×10~(-11) cm~3 molec~(-1) s~(-1). The quenching rate coeffecient of Ar gas is of order of 10~(-13) cm~3 molec~(-1) s~(-1).
The first 193 nm photon is absorbed by NH_3(X ~1A_1′) and makes it to NH_3(Ã). The predissociation of NH_3(Ã ~1A_2″) is very rapid. The nascent NH_2(Ã ~1A_2″) can further absorb a second 193 nm photon to the NH_3(B ~2B_2) state. The bond angle changes tremendously from 141.4° to 50° for a above process. The fragment NH(A ~3Π) species then possesses a high rotational excitation population which agrees the experimental temperature of T_R=7700 K.