Controlling isomer population using a dual-oscillator infrared free-electron laser

Abstract

We report on the control and characterization of the isomer population of ions inside superfluid helium nanodroplets, using two-color operation of a dual-oscillator infrared free-electron laser. The timing of both lasers is highly synchronized, and their frequencies (or "colors") can be tuned independently over a wide range. Interaction of the singly deuterated proton-bound dimer of dihydrogen phosphate and formate inside helium nanodroplets with both colors enables the control over its isomer population and the recording of - one-color hidden - infrared spectra of individual isomers.

Figures (2)

• Figure 1: Scheme of the vibrational energy levels for the two possible singly deuterated isomers of the proton-bound dimer of dihydrogen phosphate and formate that can be reached via IR allowed transitions from the vibrational ground state. The calculated, distinct IR spectra for both isomers are shown along the vertical energy axis, pointing to the left for isomer 1 and to the right for isomer 2. The geometric structures of both isomers are shown on top. The excitation, stimulated emission, and relaxation rates that are relevant when one IR-FEL (processes indicated in black) and both IR-FELs (adding the processes indicated in red) are used are indicated.
• Figure 2: IR spectra of the singly deuterated proton-bound dimer of dihydrogen phosphate and formate in helium nanodroplets, obtained by recording the signal of the bare anion as a function of the frequency of the mid-IR FEL. In the upper panel, a), only the mid-IR FEL is used. In panels b) and d), the far-IR FEL is kept fixed at 900 cm$^{-1}$ and 952 cm$^{-1}$, respectively, while scanning the mid-IR FEL. The spectra calculated at the B3LYP-D3(BJ)/aug-cc-pV(T+d)Z level of theory in the harmonic approximation of isomers 1 and 2 are displayed in panels c) and d) for comparison.