Photofragmentation of ammonia at 193.3 nm: Bimodal rotational distributions and vibrational excitation of NH2(Ã)

Abstract

Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(Ã 2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(Ã) states with rotational motion about the a-axis and without bending excitation, υ2′=0. A bimodal N′=Ka′ rotational state population distribution is observed with up to Ka′=7 in υ2′=0 and with maxima at Ka′=5 and Ka′=1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(Ã) with bending excitation, υ2′=1 and 2, is detected; the υ2′=1, N′=Ka′ rotational state population distribution spans from Ka′=0 to the energetic limit of Ka′=4. The vibrational energy partitioning for the formation of NH2(Ã,υ2′=0):NH2(Ã,υ2′=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã,υ2′=2) population is ∼10% of the total NH2(Ã) photofragments. Emission from rotational states with N′>Ka′ (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(Ã) b/c-axes rotational temperature of ∼120 K is higher than that expected from the rotationally cold parent species and is attributed to a mapping of the zero-point bending motion in the ν4 H–N–H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,υ2′,N′,Ka′)+H photofragments.