Femtosecond photodissociation dynamics of HNO[sub 3] after exitation of the S[sub 3] -state at 200 nm.

Abstract. The photodissociation dynamics of HNO[sub 3] in the electronic S[sub 3] (2[sup 1]A') state leading to the fragments OH and NO[sub 2] was investigated in real time. HNO[sub 3] was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO[sub 2] by three-photon ionization). A 2 [sup 1]A' lifetime of 60-80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO~ leading to products NO(X [sup 2]PI) and O([sup 3]P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO[sub 2] from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO[sub 2]. [ABSTRACT FROM AUTHOR]