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Photofragmentation of Ammonia 193.3 nm: Bimodal Rotational Distribution and Vibrational Excitation of NH2( )



R A. Loomis, J P. Reid, S R. Leone


Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(A2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(A) states with rotational motion about the a-axis and without bending excitation 1/2=0. A bimodal N'=K'a rotational state population distribution is observed with up to K'/a =7 in 1/2 = 0 and with maxima at K'/a = 5 and K'/a = 1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(A) with bending excitation, Ņ'/2 =1 and 2, is detected; the Ņ'/2=1, N' = K'/a rotational state population distribution spans from K'/a = 0 to the energetic limit of K'/a=4. The vibrational energy partitioning for the formation of NH2(A,Ņ'/2=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(A, Ņ'/2=2)population is 10% of the total NH2(A) photofragments. Emission from rotational states with N'>K'/a (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(A) 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 Ņ6d4^ H-N-H scissors bending coordinate of the NH3(A) predissociative state onto the NH2(A, Ņ'/2, N', K'/1) + H photofragments.
Journal of Chemical Physics
No. 2


photodissociation, reaction dynamics, zero-point motion


Loomis, R. , Reid, J. and Leone, S. (2000), Photofragmentation of Ammonia 193.3 nm: Bimodal Rotational Distribution and Vibrational Excitation of NH<sub>2</sub>( ), Journal of Chemical Physics (Accessed June 4, 2023)
Created December 31, 1999, Updated October 12, 2021