Control of the fragmentation of excited ammonia clusters by femtosecond infrared laser pulses

Applying photoion and coincident photoelectron detection in femtosecond pum p-probe experiments, we have studied the change of the fragmentation behavi or of ammonia clusters excited by femtosecond (fs) laser pulses at 200 nm t o the electronic A state which absorb an additional fs control photon 1-2 p s after the pump photon. Only a few 100 fs after the primary excitation, th e (NH3)(n) clusters are partially transferred to the vibrationally highly e xcited H-transfer state (NH3)(n-2)NH4NH2 with a lifetime of a few ps. By ir radiating the clusters in this state with control photons of a wavelength i n the range of 1200-1400 nm, we were able to excite the clusters resonantly to the next higher electronic state in the H-transfer configuration with a strongly reduced vibrational energy. The excited H-transfer state correspo nds to the 3s --> 3p transition in the NH4 component of the internally hydr ogenated clusters. Due to the strong reduction of the vibrational energy af ter the control photon absorption, the fragmentation probability in the exc ited H-transfer state and correspondingly in the ionic proton transfer stat e is drastically reduced. For example, for the ammonia dimer the signal rat io of [(NH3)(2)(+)] to [NH4+] has been enlarged by nearly one order of magn itude by the resonant control photon absorption. Whereas the lifetime of th e ammonia clusters in the nonexcited H-transfer state is nearly identical f or all cluster sizes (2-4 ps) we found distinct lifetimes tau (6) for the e xcited H-transfer state of the dimer and the trimer. For the dimer a lifeti me tau (6)=130 +/- 50 fs has been obtained for undeuterated as well as for deuterated ammonia molecules. In contrast, for the trimer the lifetime tau (6) is significantly larger and depends on the control wavelength as well a s on the isotope composition. (C) 2001 American Institute of Physics.