David M. Larsen

Excitation of polyatomic molecules by radiation

Abstract An elementary procedure for calculating quantum mechanically the time-dependent rotation-vibration wave function for a collisionless model of a polyatomic molecule in a monochromatic radiation field is described and applied to some very simple cases. It is concluded that molecules can in principle be strongly and selectively excited at radiation intensities which are too low to produce appreciable excitation in classical calculations. The excitation process is a coherent multi-photon Rabi precession between two discrete levels followed by a transition to a quasi-continuum of vibration-rotation states.

Shallow donor levels of InSb in a magnetic field

Abstract We consider, in the effective mass approximation, the problem of a hydrogenic atom in a strong static magnetic field. The ground state and the lowest lying states of angular momentum ± ħ are calculated variationally, first for a simple parabolic conduction band and then for the nonparabolic conduction band of InSb. Both the use of improved variational trial functions and the incorporation into the theory of the nonparabolic conduction band are novel features of the present calculation. The results are compared with previous work. Nonparabolic effects are found to be significant for the energy of the lowest lying state of angular momentum +ħ. The weak-coupling polaron theory is used to derive an upper bound to the shift of the ionization energy of the ground state of the impurity due to electron-LO phonon interaction. Polaron effects on the ground state ionization energy are thereby shown to be negligible in InSb. The bound obtained may, however, prove useful in estimating polaron effects in materials with larger coupling constants.

Precision verification of effective mass theory for shallow donors in GaAs

Abstract The observed Zeeman splitting of 2p-1 and 2po donor states in GaAs in the magnetic field range 20–55 kG is accounted for by the effective mass theory for isolated hydrogenic donors to an accuracy of 0.15 per cent. This is nearly an order of magnitude more precise verification of effective mass theory for donor excited states than has previously been reported in any semiconductor material.

Frequency dependence of the dissociation of polyatomic molecules by radiation

Abstract A procedure is described for calculating the dependence of the collisionless dissociation probability of a polyatomic molecule on the frequency of the laser pulse impinging upon it. Sample calculations performed on a simple molecular model show how coherent multiphoton transitions in the vibrational excitation spectrum can be observed in the curve of dissociation probability versus laser frequency.

Exciton levels in a magnetic field

Abstract A variational calculation for the first six levels of m =0 even parity states of hydrogenic system in a magnetic field is given. Our calculation, which bridges the gap between low field states and high field states, strongly supports the non-crossing rule and stands completely against either of the proposed conservation criteria for the connection of high field states to the low field states. Plots showing the evolution of the nodal surfaces of excited state wavefunctions with changing magnetic field are presented. The relevance of our results to the interpretation of experimental excition spectra in TlCl and TlBr and donor spectra in GaAs is pointed out.

Magneto-optical study of shallow donors in transmutation-doped GaAs☆

Lineshapes and peak positions of ls→2p-1 donor transitions in epitaxial GaAs samples of relatively low compensation have been studied as functions of magnetic field by use of photoconductivity measurements. Some of these samples were produced by transmutation doping using thermal neutrons—a method which is useful for the controlled introduction of donor impurities in GaAs. Two new effects, tentatively attributed to van der Waals interactions between neutral donor atoms, are observed: (1) although both Se and Ge donors are introduced by thermal neutron transmutation, the Se line is much broader than the Ge line, and (2) deviations from isolated donor behavior occur in the magnetic field dependence of the chemical shift of the shallowest donor present. The separation of lines from two deeper donors, Ge and Si, verified the simple phenomenological theory of the magnetic field dependence of central cell corrections of isolated donors up to at least 10 T.

Stark effect on donor transitions in high-purity GaAs

Abstract A quantitative study of line narrowing of the 1 s −2 p - transitions in hydrogenic GaAs donors with increasing magnetic field shows that the observed widths are due to Stark-effect inhomogeneous broadening. From the parameters involved we conclude that internal electric fields should very often play a dominant role in determining lineshapes of shallow impurity optical transitions in high purity partly compensated semiconductors at low temperatures.

Quantum well GaAs/AlGaAs shallow-donor far-infrared photoconductors grown by molecular-beam epitaxy

Photoconductors utilizing planar‐doped silicon shallow donors in GaAs quantum wells formed with AlGaAs barriers have been fabricated and measured to have far‐infrared (FIR) resonant‐wavelength responsivities of ∼1 V W−1, with noise equivalent power values of ∼1×10−7 W Hz−1/2 at 4 K. The technology makes possible the use of FIR photoconductive magnetospectroscopy to measure the well position of sheet‐doped silicon ions when incorporated at doping levels below 1×1016 cm−3. A comparison of the measured position of the ions with the intended position reveals a discrepancy that can be linked to the growth parameters used to produce the structure.

Polaron zeeman effect of shallow donors in CdTe

Abstract Far infrared Fourier transform spectroscopy has been employed to study the Zeeman splitting of the 1 s to 2 p shallow donor impurity transition in CdTe. Polaron effects are observed and are quite well accounted for by Wigner-Brillouin perturbation theory.

ODD parity donor levels of germanium in a magnetic field

Abstract The lowest three odd parity states of Ge donors are calculated in the effective mass approximation as functions of magnetic field strength for fields in the (111) and (100) directions. This extends our earlier calculations of 1s and 2s-like Ge donor states. No simplifying assumptions are introduced —we attempt to take explicitly into account the lack of cylindrical symmetry of the effective mass Hamiltonian in our choice of trial functions. Our results appear to agree reasonably well with measured transition energies of Nisida and Horii.