Structural dynamics and energy flow in Rydberg-excited clusters of N,N-dimethylisopropylamine

Abstract

In molecular beams, the tertiary amine N,N-dimethylisopropyl amine can form molecular clusters that are evident in photoelectron and mass spectra obtained upon resonant multiphoton ionization via the 3p and 3s Rydberg states. By delaying the ionization pulse from the excitation pulse we follow, in time, the ultrafast energy relaxation dynamics of the 3p to 3s internal conversion and the ensuing cluster evaporation, proton transfer, and structural dynamics. While evaporation of the cluster occurs in the 3s Rydberg state, proton transfer dominates on the ion surface. The mass-spectrum shows protonated species that arise from a proton transfer from the alpha-carbon of the neutral parent molecule to the N-atom of its ionized partner in the dimer. DFT calculations support the proton transfer mechanism between tightly bonded cluster components. The photoelectron spectrum shows broad peaks, ascribed to molecular clusters, which have an instantaneous shift of about 0.5 eV toward lower binding energies. That shift is attributed to the charge redistribution associated with the induced dipoles in surrounding cluster molecules. A time-dependent shift that decreases the Rydberg electron binding energy by a further 0.4 eV arises from the structural reorganization of the cluster solvent molecules as they react to the sudden creation of a charge.

Self-interaction corrected density functional calculations of Rydberg states of molecular clusters: N,N-dimethylisopropylamine

“…By subtracting the energy of the Rydberg states from the energy of the cation at the same level of theory, the binding energy of the Rydberg electron is obtained. The values can be compared with previously reported measurements of Deb et al 10 and Gosselin et al 5 as summarized in Table I. The agreement is excellent, within 0.1 eV.…”

“…5(a) 0.36 0.00 0.00 −0.38 Trimer, Fig. 5 10 turns out to be somewhat higher in energy, by 0.05 eV (see Fig. 2(b)).…”

“…The experimental data were previously obtained by Deb et al 10 and are reanalyzed here for the present discussion. By comparing experiments where the carrier Argon gas was at 0 • C on one hand and at −30 • C on the other hand, it can be seen that the higher temperature, which leads to more and larger clusters, shows spectra with lower binding energy of the 3s Rydberg electron.…”

“…The latter is used in the calculations of clusters because of convergence problems in the KLI when degeneracy is present. The measured values are taken from Deb et al 10 …”

“…2 In particular, the Rydberg electron binding energy has been shown to be a quite sensitive probe of molecular structure, 3 leading to the development of spectroscopic tools that use Rydberg states for the identification of molecular structure and possibly identification of molecules. [4][5][6][7][8][9][10][11][12] The technique presently provides a fingerprint of a molecular structure and is referred to as Rydberg Fingerprint Spectroscopy (RFS). It has been proven particularly useful in a time resolved photoionization setup where the kinetics and dynamics of even complex molecular reactions can be followed in real time.…”