TY - JOUR
T1 - Photofragmentation of ammonia at 193.3 nm
T2 - Bimodal rotational distributions and vibrational excitation of NH2(Ã)
AU - Loomis, Richard A.
AU - Reid, Jonathan P.
AU - Leone, Stephen R.
PY - 2000/1/8
Y1 - 2000/1/8
N2 - 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, v′2 = 0. A bimodal N′ = K′a rotational state population distribution is observed with up to K′a = 7 in v′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(Ã) with bending excitation, v′2 = 1 and 2, is detected; the v′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(Ã, v′2 = 0):NH2(Ã, v′2 = 1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã, v′2 = 2) population is ∼10% of the total NH2(Ã) 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(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 v4 H-N-H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,v′2,N′ ,K′a) + H photofragments.
AB - 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, v′2 = 0. A bimodal N′ = K′a rotational state population distribution is observed with up to K′a = 7 in v′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(Ã) with bending excitation, v′2 = 1 and 2, is detected; the v′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(Ã, v′2 = 0):NH2(Ã, v′2 = 1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã, v′2 = 2) population is ∼10% of the total NH2(Ã) 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(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 v4 H-N-H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,v′2,N′ ,K′a) + H photofragments.
UR - https://www.scopus.com/pages/publications/0001473542
U2 - 10.1063/1.480677
DO - 10.1063/1.480677
M3 - Article
AN - SCOPUS:0001473542
SN - 0021-9606
VL - 112
SP - 658
EP - 669
JO - Journal of Chemical Physics
JF - Journal of Chemical Physics
IS - 2
ER -