Collisional Excitation Transfer Research Articles

Measurement of velocity dependence of collisional excitation transfer applying resonant doppler-free 2-photon laser spectroscopy

The velocity dependence of the collisional excitation transfer process within the fine structure of the Li 3p 2P 1 2 , 3 2 levels due to the noble gases He and Ne have been measured. The relative velocities correspond to a range of 750 K (actual cell temperature) to about 5000 K. The experiment, performed in a thermionic diode, is based on the velocity selective laser excitation of Li atoms within the Doppler profile and probing the velocity group in the excited as well as the collisional populated level with a second laser. In a thermionic diode with improved sensitivity, collision processes of velocity groups can be studied which correspond to temperatures larger than 15000 K.

Excitation transfer due to neutral collisions in excited states of atomic hydrogen

The H alpha Balmer line emitted by a low-excitation electrical discharge at liquid nitrogen and room temperatures has been studied with a Fabry-Perot interferometer. With partial resolution of the fine structure, the line profile was observed to depend on buffer gas composition, pressure, and temperature, but not on discharge current. Interferograms computed from the known fine structure and transition probabilities, with allowance for non-thermal populations in the excited substates, matched the observed profiles quite well. The populations deduced from these fits are shown to approach thermal equilibrium with increasing gas pressure in a way which allows the collisional excitation transfer from the 3p to 3d states to be determined. Data for H-H, H-He, and H-H2O collisions are presented.

Excitation transfer from 31P, 31D to 33D of neutral helium by atomic collisions in a magnetic field

Laser-induced fluorescence spectroscopy is applied to a helium discharge plasma in a magnetic field: the 31P level is excited with pulsed laser light, and subsequent fluorescence intensities of the 501.5 nm (21S-31P), 667.8 nm (21P-31D) and 587.6 nm (23P-33D) lines are observed. The collisional excitation transfer rates for 31P to 33D and 31D to 33D are strongly dependent on the magnetic field: both rate coefficients obtained at 5.5 T are about 10 times larger than those at 0 T. A qualitative explanation is proposed on the basis of the mixing of spin wavefunctions between the 31D2 and 33D2 levels.

An experimental study of collisional transfer of electronic excitation, collisional dissociation of excited molecules and photodissociation in alkali vapour

Abstract A single-frequency laser is used to excite Na 2 molecules to the electronic B state. Besides the molecular fluorescence also atomic Na resonance radiation is observed. This is caused by: collisional transfer of electronic excitation from a Na 2 (B) molecule to a Na atom, collisional dissociation of Na 2 (B) molecules and photodissociation of Na 2 from very high vibration—rotation ( v. J ) levels of the ground state. We show how the contributions of these processes can be separated experimentally and characterized quantitativily over a wide range of temperatures, using a free-jet expansion. Illustrative results are given for one laser frequency (i.e. one molecular transition). Effective collision cross sections for excitation transfer and for collisional dissociation are given. The probability of photodissociation is compared to the probability of the discrete (B ← X) transition. The relaxation of the number of molecules in high ( v. J ) levels in a free jet is obtained.

Collisional energy transfer in Na 2 between the excited states B1Π u and (2) 1Σ g+: Experimental results and theoretical interpretation

Selective laser excitation and fluorescence observation techniques (Fourier transform spectroscopy) have been used to study the collisional electronic excitation transfer process: N Na 2( B 1Π u , v′, J′) + Na(3 p 2 P) → Na 2((2) 1Σ g +, v, J) + Na(3 p 3 P) + Δ E for some selected rovibrational levels of Na 2( B 1Π u ). An explicit formula is derived for the first-order, semiclassical, rotationally inelastic transition probability with an electric multipole long-range potential in the atom-dimer collision. The process is found to be resonant, with cross-sections strongly depending on Δ E; being maximum when Δ E ∼ 0 and negligible when Δ E ≳ 300 cm −1 at T ∼ 770 K. At the same time the experimentally observed spectra indicate that both resonant and nonresonant channels contribute to the collisional energy transfer. The quantitative characteristics of these processes are presented, and they are compared with calculations based on a first-order Born approximation with dipole-quadrupole long-range interactions.

Fine-structure mixing in Cs atoms excited to intermediate nD5/2states colliding with ground-state Cs atoms

The collisional excitation transfer fine-structure mixing cross sections for nD states of Cs (n=11-13) are measured by the technique of selective two-photon excitation with pulses of radiation from a N2 laser-pumped dye laser and observation of collisionally induced fluorescence.

The role of intersystem collisional transfer of excitation in the determination of N2 vibronic level populations. Application to B′ 3Σ−u–B 3πg band intensity measurements

This work constitutes a two-part study of molecular nitrogen in laboratory electric discharges. In the first part, an investigation is undertaken of the phenomenon of the intersystem collisional transfer (ICT) of excitation (recently advanced as the process which gives rise to the reddening of type B aurora) in which ICT is established as a dominant mechanism controlling the distribution of vibronic level populations under discharge conditions. In particular, it is determined that there occur many pairs of vibronic levels in adjacent electronic states in which the pair partners attain essentially identical populations through the agency of collisional coupling. In the second part, the knowledge of completely shared populations is combined with the results of a series of measurements of emission band intensities in N2 to obtain transition probabilities and transition moments for the infrared afterglow system B′ 3Σ−u –B 3πg. A tabulation of transition probabilities and vibronic level lifetimes for this system is presented.

Resonant excitation of Li atoms to the (2 2P) state by collisions with Li 2*(A 1Σ u+,υ′, J′) molecules

A cw selective laser excitation and fluorescence observation technique has been used to study the collisional electronic excitation transfer process 7Li 2 *(A 1Σ u +, υ′, J′) + Li(2 2S) → 7Li 2(X 1Σ g +, υ″, J″ = J + 1 ) + Li *(2 2P) + Δ E for some selected (υ′ = 17.6 < J′ < 30) rovibrational levels of Li 2 *(A). The process is found to be strongly resonant, with cross sections strongly depending on J′, being maximum when Δ E ≈ 0 and half-maximum when Δ E ≈ 10cm −1 at T ≈ 900 K. Measured thermally averaged depopulation cross sections of 7Li 2 *(A, υ′, J′)for Li range from 150 ± 50 A 2 for the off-resonant, J′= 27, case to 1100 ± 150 A 2 for the resonant, J′ = 21, case. The experimental results are fully consistent with semi-classical calculations based on the first Born approximation with dipole- dipole interaction.

Study of population relaxation and of collisional excitation transfer in an He-Ne laser by the nonlinear modulation spectroscopy method

An amplitude version of the nonlinear resonant modulation spectroscopy method was used to demonstrate that a strong redistribution of the channels and probabilities of collisional excitation transfer within the neon 2p multiplet, compared with those in pure neon, occurred in the discharge tube of an He–Ne laser. The following values were found for the coefficient of transfer to the 2p4 level: γ24 = 0.12, γ34 = 0.02, γ54 = 0.33. The minimum values of the detectable gain (loss) were 10 MW−1 and 0.6 MW−1 respectively for the cw and pulsed laser systems.

Intersystem collisional transfer of excitation in low altitude aurora

The characteristics of auroral optical emission as a function of altitude are exploited as a source of data for the investigation of the intersystem collisional transfer (ICT) of excitation in the nitrogen molecule. The procedure is based on a recently proposed model for the generation of the red lower border of type B auroras as resulting from the effect of the increased collision frequency at lower altitudes on the distribution of population within the excited molecules. In particular, the 85 km turn-on altitude for the red lower border of auroral arcs provides an indicator for the determination of the density and pressure required for the onset of the ICT process at mesopause temperatures. Values are obtained for collision cross sections and rate constants for the coupling of neighboring vibrational levels in adjacent electronic states in nitrogen. The results are compared with the findings of several laboratory studies on similar collisional transfers among nested electronic states. A two-part process is suggested to account for the observed features of intersystem collisional transfer phenomena.

Reactive and inelastic scattering of metastable rare-gas atoms: excitation transfer versus atom transfer

A simple analysis of the competition between the alternative channels of atom and excitation transfer in collisions of metastable rare-gas atoms with halogens is presented. The intervention of potential surfaces correlating with highly excited Rydberg states lying just below the limit of the molecular cation as well as those which correlate with the anion provides a qualitative rationale for much current experimental data.

Some spectroscopic aspects of TEA CO2laser-hydrogen glow discharge interactions

TEA CO2laser induced perturbation of shape and intensity of the first Balmer lines emitted by a hydrogen low-pressure glow discharge is investigated for radiation intensity ILin the 10 MW cm-2range. Because of the AC Stark effect, shifts of levels na=4, 5, 6, 7 and 8 are measured (i.e. for IL=8 MW cm-2, Delta E5=-0.59 cm-1, Delta E6=1.81 cm-1, Delta E7=0.55 cm-1). In addition, splitting and optical satellites originating from high orbital quantum numbers are clearly observed on the Hdeltaline. The laser one- and two-photon ionisation processes almost completely depopulate the excited states na&gt;or=8. Because of collisional radiative processes the states na=4, 5, 6 and 7 which do not undergo photoionisation are also strongly depopulated. From a kinetic model, an approximate calculation of the collisional excitation transfer rate coefficient shows that the infrared laser perturbation method can be a powerful tool for investigating collisional excitation transfers connecting excited state population in plasmas.

Mechanism for the auroral red lower border

A model is proposed for the generation of the red lower border of type B auroras that results from the effect of increased collision frequency at the lower altitudes on the distribution of excitation within the nitrogen molecule. There are two characteristic spectral distributions for the red sequence (Δυ=3) of the molecular nitrogen first‐positive system, one for normal auroras and one for laboratory (higher pressure) discharges. The laboratory spectrum is shifted in wavelength by the intersystem collisional transfer of excitation with respect to the auroral spectrum, and such a shift manifests itself to the human eye as a two‐fold increase in luminous flux in the red. Recent investigations have suggested that it is the λ5577‐Å cutoff that is responsible for the red border and have placed the lower limit of the green line emission at about 100 km. The present enhancement mechanism, however, is consistent with the onset of added bright red emission at 85 km.

Optogalvanic detection of krypton Rydberg levels with a two-step pulsed laser excitation

The authors have used an optogalvanic detection scheme, in conjunction with a two-step pulsed laser excitation, to investigate high-lying (Rydberg) krypton levels. The excitation scheme starts from the metastable 1s5 level, populated in the He-Kr discharge tube and uses either the 2p2 or the 2p3 level as an intermediate level. The transient variation of the voltage across the discharge (duration about 50 mu s) is detected as a function of the second-step laser frequency. The energy of the levels of the odd series nd(3/2)2 and nd(7/2)3 have been measured for n=15 to 25 with an accuracy of 0.03 cm-1. Members of the even series nf(7/2)J, nf(9/2)J and np(5/2)3 have also been identified and the corresponding energies measured. In the latter case, the excitation scheme involves a collisional excitation transfer from the laser populated intermediate levels to the closely lying 3d4' level.

Excitation Process of the N2 IR (B3Πg-A3Σu+) Laser Transition. II

The excitation process of the IR (B3Πg-A3Σu+) laser action in the N2 molecule is investigated by rate equation analysis of the time lag between the uv (C3Πu-B3Πg) and IR laser pulses in an axially-excited N2 laser. It is found that with an increase in pressure the main excitation process of the upper IR laser level changes from direct electron impact excitation inclusive of the cascade process due to the UV laser action to collisional excitation transfer from the a1Πg metastable state. The collisional excitation transfer dominates the excitation process for the upper IR laser level at optimum pressures for the IR output power. The time-dependent behaviors of the quantities relevant to the IR laser action in the proposed model are given in detail.

High resolution spectrum of the N_2^+ Meinel system to 11 250 Å

This spectrometric investigation of the emission of the A2Πui→X2Σg+ band system of the nitrogen molecular ion has yielded the wave numbers of 1267 individual lines in eight bands. Cooled photo-multipliers were used up to the 1-0 band at 9200 Å and a cooled intrinsic germanium photodiode was used for the 0-0 band. The spectra were obtained using a specialized hollow cathode source and a 5-m Jarrell-Ash spectrograph adapted to spectrometric operation through the installation of an exit slit translated with a lead screw assembly driven by a stepping motor. Improved molecular constants and vibrational term values have been determined as well as the origins of the 0-0 and 1-0 bands which have not previously been subject to direct measurement and which specify the bottom of the A2Πu potential curve. Observation of a very intense 0-0 band indicates a large population for the υ = 0 level of the A2Πu state under the conditions of the experiment. It is concluded that collisional transfer of excitation between electronic states is responsible for the accumulation of population in this lowest excited level of N2+.

Excitation Process of the N2 IR (B3Πg-A3Σu+) Laser Transition

Excitation mechanism of the laser action on the first positive system (B3Πg-A3Σu+) of N2 molecule is studied with an emphasis on the time relationship between the ir and uv (C3Πu-B3Πg) laser pulses. Pressure dependence of time lag between the uv and ir pulses has been observed in the pressure range from 4 to 8 Torr using an axial-discharge tube. The observed time lag is well predicted by a model which involves direct electron-impact excitation, near-resonant collisional excitation transfer from the a1Πg state, and cascade excitation due to the uv laser action for the upper ir laser level. Relative importance of these excitation processes depends primarily on N2 pressure.

Molecular infrared laser with Br*→CO2electronic–vibrational energy transfer as the pump mechanism

Results are presented of an experimental investigation of an infrared laser utilizing IBr–CO2 and Br2–CO2 mixtures pumped by electronic-vibrational collisional excitation transfer from electronically excited Br*(42P1/2) atoms to Raman vibrational states [(1001)–(0201)] of the CO2 molecule. It is shown that, under conditions of broad-band flash photolysis, the IBr molecule is a more efficient supplier of Br*(42P1/2) atoms than Br2As a result, simultaneous three-frequency laser action at wavelengths of 2.7 μ (42P1/2→42P3/2 transition in atomic bromine), 4.3, and 10.6 μ (vibrational–rotational transitions in the CO2 molecule) was obtained from an IBr–CO2 mixture. On the other hand, laser action was only obtained at 10.6 μ from a Br2–CO2 mixture. A qualitative laser model developed showed satisfactory agreement with the experiment.

Collisional excitation transfer in high magnetic fields. I. Theory

The effects of strong magnetic fields on thermal collisions in a vapor are reviewed. Semiclassical and quantum-mechanical symmetry properties of the relaxation matrix are established for resonant collisions between identical atoms and nonresonant collisions with a structureless partner. The variations with field strength of the transfer rates between Zeeman sublevels for ${R}^{\ensuremath{-}3}$ dipole-dipole interaction and ${R}^{\ensuremath{-}6}$ Van der Waals interaction are obtained with an exact semiclassical computation of the collision matrix. These are then compared with those predicted with an approximate formulation by means of the symmetrical correlation function for the potential.

Cascade lasing and collisional excitation transfer in optically pumped calcium atoms

Cascade lasing and collisional excitation transfer in optically pumped calcium atoms