Photofragmentation of ammonia at 193.3 nm: Bimodal rotational distributions and vibrational excitation of NH2((A)over-tilde)

Time-resolved Fourier transform infrared emission spectroscopy is used to m easure the nascent rovibrational distribution of low-lying electronically e xcited NH2((A) over tilde (2)A(1)) produced in the 193.3 nm photolysis of r oom-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2((A) over tilde) states with rotational motion about the a-axis and without bending excitation, upsilon(2)'=0. A bimodal N-'=K-a' rotational s tate population distribution is observed with up to K-a'=7 in upsilon(2)'=0 and with maxima at K-a'=5 and K-a'=1. We suggest that the bimodal rotation al distribution may result from the competition between planar and bent geo metries during dissociation. Weaker emission from NH2((A) over tilde) with bending excitation, upsilon(2)'=1 and 2, is detected; the upsilon(2)'=1, N' =K-a' rotational state population distribution spans from K-a'=0 to the ene rgetic limit of K-a'=4. The vibrational energy partitioning for the formati on of NH2((A) over tilde,upsilon(2)'=0):NH2((A) over tilde,upsilon(2)'=1) i s 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respective ly. An upper limit of the NH2((A) over tilde,upsilon(2)'=2) population is s imilar to 10% of the total NH2((A) over tilde) photofragments. Emission fro m rotational states with N'> K-a' (molecules with rotational excitation abo ut the b/c-axes) is also observed. Under jet-cooled conditions the NH2(A) b /c-axes rotational temperature of similar to 120 K is higher than that expe cted from the rotationally cold parent species and is attributed to a mappi ng of the zero-point bending motion in the nu(4) H-N-H scissors bending coo rdinate of the NH3((A) over tilde) predissociative state onto the NH2((A) o ver tilde,upsilon(2)',N',K-a')+H photofragments. (C) 2000 American Institut e of Physics. [S0021-9606(00)00202-6].