ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Applying photoion and coincident photoelectron detection in femtosecond pump–probe experiments, we have studied the change of the fragmentation behavior of ammonia clusters excited by femtosecond (fs) laser pulses at 200 nm to the electronic A˜ state which absorb an additional fs control photon 1–2 ps after the pump photon. Only a few 100 fs after the primary excitation, the (NH3)n clusters are partially transferred to the vibrationally highly excited H-transfer state (NH3)n−2NH4NH2 with a lifetime of a few ps. By irradiating the clusters in this state with control photons of a wavelength in 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 corresponds to the 3s→3p transition in the NH4 component of the internally hydrogenated clusters. Due to the strong reduction of the vibrational energy after the control photon absorption, the fragmentation probability in the excited H-transfer state and correspondingly in the ionic proton transfer state is drastically reduced. For example, for the ammonia dimer the signal ratio of [(NH3)2+] to [NH4+] has been enlarged by nearly one order of magnitude by the resonant control photon absorption. Whereas the lifetime of the ammonia clusters in the nonexcited H-transfer state is nearly identical for all cluster sizes (2–4 ps) we found distinct lifetimes τ6 for the excited H-transfer state of the dimer and the trimer. For the dimer a lifetime τ6=130±50 fs has been obtained for undeuterated as well as for deuterated ammonia molecules. In contrast, for the trimer the lifetime τ6 is significantly larger and depends on the control wavelength as well as on the isotope composition. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1377889
