Superfluid Helium Droplets: A Uniquely Cold Nanomatrix for Molecules and Molecular Complexes

Abstract

Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.15 K (3He droplets), lower than possible in most solid matrices. Thus the helium-droplet technique combines the benefits of both the gas phase and the classical matrix-isolation techniques. Most important, the superfluid helium facilitates binary encounters, and absorbs the released binding energy upon recombination. Thus the droplet can be viewed as an isothermal nanoscopic reactor, which isolates single molecules, clusters, or even a single reactive encounter at ultralow temperatures.

Photoionization Dynamics in Pure Helium Droplets

“…17 While many interesting effects occur under these source conditions, 17,29,31 the three most important for our experiment are that the atomic helium component of the beam appears to decrease, the average droplet size increases sharply, and the overall flux increases dramatically. 17,20,29 Examining the pressure-temperature phase diagram for helium, 17 we estimate that, with 60 bar backing pressure, the transition to the critical regime should begin near 16 K, precisely where we observe the marked transition in the total electron yield (Figure 4) and the strong changes in the photoelectron spectra ( Figure 5). The offset in temperature for the 45 bar data shown in Figure 4 is also consistent with our predictions for the onset of regime II.…”

A new sensitive detection scheme for helium nanodroplet isolation spectroscopy: application to benzene

“…The new method has the potential to obtain simultaneously spectroscopic and mass information. To explore this potential we have recorded the 6 1 0 transition of the benzene dimer in helium droplets by gating the detector at the arrival times of benzene and (benzene) 2 ions. The resulting spectra are presented in Fig.…”

“…[1][2][3][4] The low temperature of the helium droplets and the weak interactions of the helium with the solutes allows for the observation of greatly simplified spectra. None of the spectroscopic methods used in helium nanodroplet isolation spectroscopy records directly the absorption spectrum.…”

“…[5][6][7][8][9] Whereas these two techniques use direct probes of the molecular absorption, almost all other techniques rely on the non-radiative energy transfer from the optically excited molecule to the helium environment. [1][2][3][4] Following this energy transfer, the heated helium droplets cool down to their equilibrium temperature of 0.38 K by the evaporation of helium atoms from their surface, thereby reducing their size. This reduction in droplet size can be readily detected using mass spectrometry, since the cross section for electron impact ionization of droplets consisting of hundreds or more helium atoms scales with the geometrical cross section, i.e.…”

Formation of Au and tetrapyridyl porphyrin complexes in superfluid helium

“…Helium nanodroplets are large helium clusters composed of typically 10 3 -10 8 helium atoms. [18,19] These droplets have an equilibrium temperature of 0.37 K (for 4 He) and they can pick up atoms/molecules with a near unit probability. [20] When different types of atoms/molecules are added to the droplets, aggregation into binary complexes is expected.…”