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Articles 1 - 14 of 14
Full-Text Articles in Physics
Magnetic-Field-Induced Alignment-To-Orientation Conversion In Sodium, G. Schinn, Xianming Han
Magnetic-Field-Induced Alignment-To-Orientation Conversion In Sodium, G. Schinn, Xianming Han
Xianming Han
We report a detailed investigation of excited-state alignment-to-orientation conversion in the presence of an external magnetic field. This counterintuitive phenomenon occurs under intermediate-coupling conditions. A weak, linearly polarized, cw laser beam was used to excite and align the Na 3P3/2 state in an atomic beam along the z direction. The degree of circular polarization of the resulting fluorescence was detected along the z direction as a function of magnetic-field strength. The spectrally integrated transitions originating from individual F levels of the 3S1/2 state yield a maximum circular-polarization fraction of ∼40%; integrating the circular polarization over all the allowed 3S1/2-3P3/2 transitions …
Differential, Partial Cross Sections For Electron Excitation Of The Sodium 3p State, Xianming Han, G. W. Schwinn, A. Gallagher
Differential, Partial Cross Sections For Electron Excitation Of The Sodium 3p State, Xianming Han, G. W. Schwinn, A. Gallagher
Xianming Han
Using a powerful laser-based experimental method, the cross section for electron excitation of Na(3S) atoms to the Na(3P) state has been decomposed into partial components with respect to changes in the spin and angular momentum of the atomic electron, and these partial cross sections are further reduced to their differential character with respect to the electron scattering angle. Partial, differential cross sections are reported for electron collision energies from threshold 2.1 to 3.6 eV, and compared to available calculations.
Collision Induced Superfluorescence, A. Kumarakrishnan, Siddharaj Chudasama, Xianming Han
Collision Induced Superfluorescence, A. Kumarakrishnan, Siddharaj Chudasama, Xianming Han
Xianming Han
We have studied superfluorescence (SF) in Ca vapor evolving on the 3d4s3DJ-4s4p3PJ−1 transitions at 1.9 mm by exciting the 4s21S0-4s4p1P1 with a pulsed dye laser. SF is generated following population transfer by spinchanging collisions with an inert gas Ar from the 4s4p1P1 and 3d4s1D2 levels. We show for the first time to our knowledge that the time delay for SF evolution follows the 1/ÎN dependence expected for the case of uniform excitation of the vapor column by collisional transfer. Here, N is the number of participating atoms that was measured directly from the photon yield. The measured photon yield for …
Superfluorescence Polarization: Signature Of Collisional Redistribution, A. Kumarakrishnan, S. Chudasama, Xianming Han
Superfluorescence Polarization: Signature Of Collisional Redistribution, A. Kumarakrishnan, S. Chudasama, Xianming Han
Xianming Han
We have studied effects of magnetic sublevel degeneracy on the polarization of superfluorescent pulses generated on the Ca 4s4p1P1–3d4s1D2 transition at 5.5μm. These pulses were generated from a cell of length 50 cm by optically pumping calcium vapor on the 4s21S0–4s4p1P1 transition in the presence of Ar gas. The axis of ellipticity of superfluorescence (SF) polarization is oriented parallel to the axis of the pump-laser polarization at large detunings, and undergoes an abrupt rotation through 90° for detunings close to resonance. The distribution of populations in the magnetic sublevels of the 1P1 state can be estimated using a simple model …
Diamond Growth Reactor Chemistry And Film Nucleation Enhancement Using Chlorinated Hydrocarbons, James Chenault, Charles Feigerle, Xianming Han, Robert Shaw
Diamond Growth Reactor Chemistry And Film Nucleation Enhancement Using Chlorinated Hydrocarbons, James Chenault, Charles Feigerle, Xianming Han, Robert Shaw
Xianming Han
The chemistry of diamond film growth from chlorinated hydrocarbons has been investigated using a hot filament reactor coupled to an orifice sampling mass spectrometer. The relative concentrations of the species present near the growth surface have been determined as a function of filament temperature for dilute mixtures of CH4, CH3Cl, CH2Cl2 and CHCl3 in H2. Mass spectral analysis indicated that chlorinated hydrocarbons are sequentially dechlorinated in the presence of hydrogen at moderate reactor temperatures. A dark film was deposited on all surfaces of the reactor during studies of this dechlorination of CHCl3. Raman analysis indicated that these deposits are small …
Fine‐Structure Mixing Within The Zn(43pj) Multiplet By Collisions With The Noble Gases, Xianming Han, J. F. Kelly
Fine‐Structure Mixing Within The Zn(43pj) Multiplet By Collisions With The Noble Gases, Xianming Han, J. F. Kelly
Xianming Han
Measurements of rate coefficients for intramultiplet state transfer of Zn(4 3 P 1→4 3 P J’) by collisions with the rare gases are presented. The state‐to‐state binary rate coefficients are derived from least‐squares fittings of the time‐resolved triexponential behavior of the 4 3 P 1fluorescence. These rate coefficients were studied systematically over a temperature range of 690–1100 K in order to characterize the velocity dependence of the collisional coupling. The systematic behavior of the rate coefficients with varying temperature and noble gas species is qualitatively consistent with a nearly adiabatic coupling limit for noncrossing levels.
Collisional Dynamics Of The First Excited States Of Neon In The 590-670 Nm Region Using Laser Optogalvanic Spectroscopy, Xianming Han, M. Su, C. Haridass, P. Misra
Collisional Dynamics Of The First Excited States Of Neon In The 590-670 Nm Region Using Laser Optogalvanic Spectroscopy, Xianming Han, M. Su, C. Haridass, P. Misra
Xianming Han
A mathematical rate equation model, incorporating the various processes contributing to the generation of optogalvanic signals in a discharge plasma, has been used to analyze the time-resolved waveforms of neon in the wavelength region 590–670 nm. Amplitudes, decay rates and the appropriate instrumental time constant have been determined using a non-linear least-squares fit of the observed time-resolved optogalvanic waveforms.
Spin-Exchange Cross Section For Electron Excitation Of Na 3s 3p Determined By A Novel Spectroscopic Technique, Xianming Han, G. W. Schwinn, A. Gallagher
Spin-Exchange Cross Section For Electron Excitation Of Na 3s 3p Determined By A Novel Spectroscopic Technique, Xianming Han, G. W. Schwinn, A. Gallagher
Xianming Han
An experimental technique is described which enables determination of the partial cross sections for electron excitation of atoms as a function of changes in spin and orbital angular momentum. This method provides a good signal-to-noise ratio in the energy region near threshold, and could be used to study long- or short-lived excited states of many atomic systems. Measurements for Na 3S−3P, near-threshold excitation are reported here. The results are generally in good agreement with the close-coupling calculations of D. L. Moores and D. W. Norcross [J. Phys. B 5, 1482 (1972)] for the largest ΔmS and ΔmL components of the …
Monte Carlo Least-Squares Fitting Of Experimental Signal Waveforms, Xianming Han, Vladimir Pozdina, C. Haridas, Prabhakar Misra
Monte Carlo Least-Squares Fitting Of Experimental Signal Waveforms, Xianming Han, Vladimir Pozdina, C. Haridas, Prabhakar Misra
Xianming Han
This paper focuses on why the regular least{squares ¯tting technique is unstable when used to ¯t exponential functions to signal waveforms, since such functions are highly correlated. It talks about alternative approaches, such as the search method, which has a slow convergence rate of 1=N1=M, for M parameters, where N is the number of computations performed. We have used the Monte Carlo method, utilizing both search and random walk, to devise a stable least{squares ¯tting algorithm that converges rapidly at a rate 1=N1=2, regardless of the number of parameters used in ¯tting the waveforms. The Monte Carlo approach has been …
Superfluorescence From Optically Trapped Calcium Atoms, Xianming Han
Superfluorescence From Optically Trapped Calcium Atoms, Xianming Han
Xianming Han
We have studied superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler-broadened medium. Nanosecond SF pulses at 5.5 μm were generated on the Ca 4s4p1P1–3d4s1D2 transition from a column of calcium vapor buffered with Ar by optically pumping the 4s21S0–4s4p1P1 transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our …
Abrupt Change In Neon Discharge Plasma Detected Through Optogalvanic Effects, Xianming Han, Michael Blosser, Prabhakar Misra, Haridass Chandran
Abrupt Change In Neon Discharge Plasma Detected Through Optogalvanic Effects, Xianming Han, Michael Blosser, Prabhakar Misra, Haridass Chandran
Xianming Han
When a laser is tuned between two excited energy levels of a gas in a Direct Current discharge lamp, the discharge current will experience a temporary disturbance lasting tens or hundreds of microseconds known as the optogalvanic effect. We have carried out extensive studies of optogalvanic effects in neon discharge plasmas for transitions at 621.7 nm, 630.5 nm, 638.3 nm, 650.7 nm and 659.9 nm. A nonlinear least-squares Monte Carlo technique has been used to determine the relevant amplitude coefficients, decay rates and the instrumental time constant. We discovered an abrupt change in the neon discharge plasma at a discharge …
Collisional Ionization Of Excited State Neon In A Gas Discharge Plasma, Xianming Han, Verl Wiseheart, Scott Conner, Meng-Chih Su, David Monts
Collisional Ionization Of Excited State Neon In A Gas Discharge Plasma, Xianming Han, Verl Wiseheart, Scott Conner, Meng-Chih Su, David Monts
Xianming Han
We report the first case where it is possible to clearly identify and quantitatively characterize the dominant physical processes contributing to production of the optogalvanic effect (OGE) signal in a discharge plasma. This work concentrates on the simplest case where only two states are involved in the optical transition. The theoretical model with only four parameters is in excellent agreement with the experimentally obtained time-resolved OGE waveforms. The collisional ionization rate in the upper state is twice as fast as that in the lower state although the two states are only separated by 1.94 eV. We conclude that the optogalvanic …
Investigations Of Superfluorescent Cascades, A. Kumarakrishnan, Xianming Han
Investigations Of Superfluorescent Cascades, A. Kumarakrishnan, Xianming Han
Xianming Han
We report our studies of superfluorescent cascades in atomic calcium which result from two-photon excitation of several levels reasonably close to the ionization limit. We have observed significant conversion efficiencies for some of these transitions which result in subnanosecond pulses particularly in the visible wavelengths. We report the discovery of a novel two-photon scattering mechanism which could prove to be a useful method for determining collisional broadening rates. In addition, a hyper Raman transition near 17 μm is discovered which appears to be a promising candidate for a tunable source.
Production And Diagnosis Of A Highly Spin-Polarized Na Beam, G. Schinn, Xianming Han, A. Gallagher
Production And Diagnosis Of A Highly Spin-Polarized Na Beam, G. Schinn, Xianming Han, A. Gallagher
Xianming Han
We describe optically pumping a beam of sodium atoms to >96% mS and >92% mS, mI state selection. (We have accurately measured the population of every mS, mI state in the optically pumped beam.) For the optical pumping both ground hyperfine states are pumped, using single-mode cw dye-laser radiation tuned to the 3S1/2–3P1/2 transition that is phase modulated in a LiTaO3 crystal to produce first-order sidebands at approximately the 1772-MHz hyperfine splitting of the ground state. The z-directed optical pumping is performed in a z-directed magnetic field of ~5 G. The state-selected atoms then move, in ~1 cm, into an …