PHOTODISSOCIATION DYNAMICS OF (A)OVER-TILDE STATE AMMONIA MOLECULES .2. THE ISOTOPIC DEPENDENCE FOR PARTIALLY AND FULLY DEUTERATED ISOTOPOMERS

The technique of H(D) Rydberg atom photofragment translational spectro scopy has been used to investigate the photodissociation dynamics of t he mixed isotopomers NH2D and NHD2 following the excitation to the nu' (2) = 0 and 1 levels of their lowest lying (A) over tilde(1)B(1) (C-2 nu) excited electronic states. Peaks in the resulting total kinetic en ergy release (TKER) spectra are assigned to levels of the NH2, NHD, or ND2 fragments with a wide range of quantum numbers K-a for rotation a bout their a inertial axes, and with N=K-a, N=K-a+1, or N=K-a+2 as app ropriate. These data provide the first measurements of high rotational levels for the ground electronic state of the NHD radical. The least squares fitting of all these spectra, and those resulting from NH3 and ND3, to the best calculated NH2, NHD, and/or ND2 rotational term valu es provides accurate estimations of the respective N-H and N-D bond di ssociation energies D-0(0) the whole series. These values are D-0(0)(H -NH2)=37 115+/-20 cm(-1) (4.602+/-0.002 eV); D-0(0)(H-NHD)=37 240+/-50 cm(-)1; D-0(0)(H-ND2)=37 300+/-30 cm(-1). D-0(0)(D-NHD)=37 880+/-60 c m(-1); and D-0(0)(D-ND2)=38 010+/-20 cm(-1). The differences between t hese values are fully consistent with differences in zero-point cm ene rgies and lead to a mean value of D-e=40 510+/-25 cm(-1). Dissociation of NH2D or NHD2 through their ((A) over tilde-(X) over tilde 2(0)(1), bands to give an NHD product leads to TKER spectra with a much higher statistical character than those leading to an NH2 or ND2 product, an d to those obtained following excitation through the 0(0)(0) bands. Th is is rationalized in a semiquantitative manner in terms of a varying contribution to the dissociation rate of the parent molecules from int ernal conversion (IC) to high levels of their respective ground states . Nuclear permutation symmetry appears to play an important role both for the IC rates and for the subsequent branching between product chan nels. (C) 1996 American Institute of Physics.