Ultrafast photodissociation dynamics and energetics of the electronically excited H atom transfer state of the ammonia dimer and trimer

摘要

The energetics and ultrafast dynamics in the H atom transfer configuration of ammonia dimer and trimer clusters have been studied. The clusters are first excited to the electronic A state with a 208 nm femtosecond laser pump puls. This state is allowed to relax for about 1 ps during which the H-transfer state is formed which is then electronically excited by a time-delayed infrared control pulse at 832 nm and finally ionized with a third femtosecond probe pulse at 416 nm. We have also performed complementary theoretical studies elucidating the experimental findings. For the dimer in the excited NH_4(3p)…NH_2(X) state the time-dependent ion signals reveal an isotope-independent short lifetime of about #tau#_6=(30+-60) fs which can be explained by a curve crossing with the repulsive NH_4(3s)…NH_2(A) state, whereas the trimer signal persists on a time scale being more than one order of magnitude longer and exhibits a very large isotope effect. This is interpreted as being due to internal conversion from the excited state NH_3NH_4(2p)…NH_2(X) back to the NH_3NH_4(3s)…NH_2(X) ground state. The analysis of the corresponding photoelectron spectra also confirms the transition energies between the electronic state involved, e.g., #DELTA#E[NH_4(3s->3p)…NH_2]=1.5 eV and #DELTA#E[NH_3NH_4(3s->3p)…NH_2]=1.2 eV, as determined by our ab initio calculations.