Matrix Isolation by Solid Helium

Eugene B. Gordon Russian Academy of Science Institute of Chemical Physics Problems, Quantum Systems Laboratory Chernogolovka, 142432 Moscow region, Russia.

Helium is the most inert chemical element and that is the motive to put the spectroscopic objects into low-perturbed helium matrix. Meanwhile the extreme weakness of interatomic interaction manifesting itself in extraordinary flexibility of solid and liquid helium causes the noticeable perturbation of the matrix by an embedded impurity itself. Besides its demonstration in the impurities spectra such an effect should be detectible by changes in the specific for helium quantum effects such as delocalization and superfluidity. There are a few of elegant experimental approaches to the impurity introduction into condensed helium. Very popular is the technique of impurity capture to cold liquid helium droplets [1]; the intriguing results have been obtained by laser ablation from the target placed inside liquid and, especially, solid helium [2]. Here we will be mainly concentrated on the method based on condensation of gas helium jet contained the small admixture of particles under study. Its obvious advantages are the possibilities of large impurity densities achievement and of changing both temperature and pressure; that allows revealing the effect of impurities affecting the quantum matrix. This approach meets evident difficulty: every “hot” (300K) helium atom releases under its condensation the energy of 450K whereas every evaporated atom takes away only 7K. It means the helium outflow should be 65 times stronger than the inflow and thus no flow should exist in one-dimensional case. The solution found in 1974 was to separate the flows in a space: originated from small orifice the pointed gas He jet carried the impurity to the surface of superfluid helium cooled by its evaporation, supersonic jet velocity protected gas from cooling and the admixture from coalescence [3]. Recently the technique has been modified by physical separation the inflow and outflow, the last simply cooled down the experimental cell by thermoconductivity through its wall [4]. The absence of counterflow resulted in significant enhance of probability for impurity to be captured inside of liquid He. Briefly the effect of He surrounding on electronic transitions in atoms can be demonstrated by example of the forbidden 2DÆ4S transition in nitrogen atoms. On one hand, helium practically doesn’t quench the excitation (the radiative time is as long as in a gas, t = 104 s) but, on the other hand, the atomic line transforms to broad, 50 cm-1, band. Moreover ESR study shown the absence of N atoms mutual recombination in a sediment formed in superfluid helium. That led us to the alluring idea of Impurity-Helium Solid – the metastable substance consisted of frozen together localized helium clusters [5]. However, in spite of many efforts there have not yet been unambiguous proofs of its real existence. That challenge, as well as the problems of superfluidity fingerprints in rotation structure of IR spectra for molecules isolated in liquid 4He and of strong difference in magnetic relaxation times for alkali atoms isolated in bcc and hcp phases of solid He, stimulated us to develop the technique for impurity condensation directly in solid helium. It consists in injecting high pressure He gas mixed with an impurity on the top surface of liquid-solid interface, with the solid continuously moving downwards by pumping at the bottom of the cell; the sedimentation occurs through the thin (2 mm) upper layer of liquid helium. We achieved a guest- particle density of 3⋅1019 per cm3 and a doped crystal growth rate of 0.05 mm/s. The results of CARS study of small deuterium clusters isolated by SHe have shown that the effect of large predominance of Q1(1) transition over Q2(0) one expressed in CARS even more than in the common Raman scattering. The significant size effect has been revealed there when the size of cluster became to be close to scattered light wavelength [6]. The preliminary results of IR spectroscopy of simple molecules embedded into SHe will be reported, the special attention will be paid to temperature and pressure dependences and to their transformation under the doped solid temporary melting. 1. S.Goyal, D.L.Schutt, G.Scoles, PRL, 69, 933(1992); J.P.Toennies, A.F.Vilesov, Annu. Rev. Phys. Chem., 49, 1 (1998) 2. S.I. Kanorsky, A. Weis, Adv. Atomic, Molecular & Optical Physics, 38, 87 (1987); K.Ishikawa, A.Hatakeyama, K.Gosyono-o, et al., Phys.Rev. B56(2),780 (1997) 3. E.B. Gordon, L.P. Mezhov-Deglin, O.F. Pugachev, JETF Lett. 19, 63 (1973) 4. R.E.Boltnev, G.Frossati, E.B.Gordon, et al., JLTP 127(5/6),247 (2002) 5. E.B.Gordon, V.V.Khmelenko, A.A.Pelmenev, et al., CPL 155, 301 (1989) 6. E.B.Gordon, G.Frossati, A.Usenko, et al, Physica B (accepted)