Ion–ion coincidence measurements on H2O, HDO, and D2O ionized by Heii light show that the main ion-pair dissociations from low-lying states of H2O2+ lead to OH++H+ and H++H++O. Distinct states of H2O2+ are deduced to be at 36.5±1 and at 42±1 eV above the H2O ground state. Double charge transfer measurements show two sets of singlet states of H2O2+ at 41.4 and 45 eV in agreement with the Auger spectrum. New ab initio calculations predict a vertical appearance energy of 39.2 eV for the H2O2+ ion in its 3B1 ground state, but show a very strong geometry dependence, and give an adiabatic value near 36.3 eV, in agreement with experiment.
Translational energy spectra of H− ions arising from double charge transfer to 1.5–7.0 keV protons from methane have been recorded and interpreted as the single and double ionization spectra of CH4. The excited states of CH++4 are not resolved, but a large broad peak of 4 eV half-width is observed, starting at around 35 eV, with a long tail extending to 44 eV. The peak maximum is at 38.6 eV. Four dissociation reactions of CH2+4 have been observed using photoion–photoion coincidence techniques with Heii light. All the principal discrepancies in comparisons between the experimental and theoretical work on CH2+4 can now be reconciled.
The magnetoconductivity o (8) of two-dimensional electrons on liquid helium was measured above 1 K, using high-precision Corbino electrodes. In low magnetic fields 8, o(0)/o(8) = I + (tI, B)2 as in the Drude model, where p, is the zero-field mobility due to scattering by He vapor atoms, even for pB» , 1. At higher fields fr(0)/o(8) becomes density dependent due to ffiany-electron effects, in excellent agreement with the theory of Dykman. Only at the highest fields, or the lowest densities, does a(B) approach the theoretical single-particle magnetoconductivity. PACS numbers: 73.20.Dx, 67.90.+z, 73.50.Jt Electrons above the surface of superfluid helium form a nearly ideal two-dimensional electron system (2DES).At the electron densities, n & 2 X 10" m 2, which are stable on bulk liquid, the electrons are nondegenerate and are perhaps the best example of a classical 2D conducting system [1]. The electrons are held in a vertical potential well produced by the repulsion of the helium surface and an applied vertical electric pressing field.At low temperatures T they are in the quantum ground state for vertical motion but are free to move horizontally. The electrons are scattered by 4He vapor atoms above 1 K and by surface ripplon modes on the helium at lower temperatures where the zero-field mobility p, can be as high as 103 m2/Vs. The matrix elements for the scattering are well known [2] and give good agreement with the experimental values for p, (T) [3]. Vapor atoms act as almost ideal point-scattering centers with quasielastic scattering (due to the large mass ratio). The application of a vertical magnetic field 8 to this system is particularly interesting for pB) I, t, he region of classically strong magnetic fields. In the singleelectron approximation the energy spectrum becomes a series of discrete quantized Landau levels, energy separation liat"where ay, = eB/m is the cyclotron frequency.The extreme quantum limit, boy, /kT = 1.3448/T ) 1, in which only the lowest I uidau level is occupied, is easily reached. However, the Coulomb interaction between electrons gives a fluctuating many-electron electric field which changes the discrete Landau levels into a continuous spectrum and strongly influences the magnetotransport in the quantum region. This was first recognized by Dykman who gave theoretical expressions for ripplon [4,5] and vapor atom scattering [6]. The Drude model for the magnetoconductivity o (8) of a classical 2DES assumes classical cyclotron orbits [radius R, = (2mkT)'/~/eB] and a field-independent scattering rate and gives CFp 1 2 -,.(.) = -(. ) =''~"' where frp = o'""(0) = nehru, is the zero-field conductivity.It is equivalent to the Einstein relation fr"= ne~L2/kTr where L is the diffusion length (~orbit radius in high fields) and I/r is the scattering rate. The formation of Landau levels changes the energy density of states and hence the scattering rate becomes field dependent. The quantum cyclotron radius must also be used in quantizing magnetic fields. Both these effects are included in the orb...
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