Attosecond dynamics of nuclear wavepackets induced by neutron Compton scattering

For the first time, time-dependent nuclear wavepacket theory is applied to the experimental context of neutron Compton scattering (NCS). The derivation is analogous to the well-known expression of infrared laser absorption spectra (IR-LAS) in terms of autocorrelation functions of nuclear wavepackets moving on molecular potential energy surfaces in the electronic ground state. This analogy allows us to transfer the methods for nuclear wavepacket dynamics from IR-LAS to NCS. Systematic investigations for two model systems, HOD and C6D5H, demonstrate the effects of nuclear dynamics induced by NCS in the as (10(-18) s) time domain on the NCS spectra. This is a consequence of the large momentum transfer from the neutron to the scattering atom and consequentially the ultrashort time for the nuclear wavepacket to travel the distance of its narrow width, followed by dissociation. This initial time evolution may be described approximately in terms of normal mode vibrations, together with additional excitations of translations and rotations which support depletion of any recurrences of the vibrational autocorrelation functions, also due to dissociation. In spite of the analogous derivation we predict some surprising, opposite trends in NCS i.e. in contrast to LAS. Thus, increasing the number of excited modes for polyatomic molecules, the resulting dynamics slow down for NCS and therefore, the spectral width narrows. (C) 2004 Elsevier B.V. All rights reserved.