Collision-induced Transitions Research Articles

Collision-induced transitions between A 1Σu+ and b 3Πu states of Na2: The ‘‘gateway’’ effect of perturbed levels

We present here the best qualitative and quantitative illustration to date of the perturbation ‘‘gateway’’ effect in collision-induced transitions between two mutually perturbing electronic states. The gateway effect, as described by Gelbart and Freed [Chem. Phys. Lett. 18, 470 (1973)], is a suggestion that all collision-induced transfer of population between two electronic states proceeds through a small number of isolated-molecule eigenstates which are of mixed electronic character, the ‘‘gateway levels,’’ and that the rates for such gateway-mediated processes are related to the mixing fractions in the gateway levels. The gateway levels here are the Na2 A 1Σu+ v′=26∼b 3Π2u v′=28 J′=16e,a-symmetry levels which are significantly mixed owing to an extremely small spin–orbit perturbation matrix element (the neighboring J′=15 and 17e,s-symmetry levels are essentially free of mixing). A cw optical–optical double resonance (OODR) scheme is used to PUMP a single parent level and PROBE single daughter and granddaughter levels. The oscillator strengths for the PUMP and PROBE transitions are derived, respectively, from the A 1Σu+←X 1Σg+ (26,4) band and the 2 3Π2g←b 3Π2u (28,28) subband. The qualitative observation of the gateway effect is that whenever an a-symmetry A 1Σu+ v′=26 parent level is selected, b 3Π2u v′=28 daughter and granddaughter levels are observably populated, but when an s-symmetry A 1Σu+ v′=26 parent is selected, essentially no population is detected in b 3Π2u v′=28 daughter and granddaughter levels (i.e., no perturbation, no interelectronic state transfer). The quantitative observation of the gateway effect is that when a J′=12 (or 14)e,a parent is selected, the most efficiently populated rotational levels of the other electronic state are granddaughter levels centered about the J′=16e,a gateway daughter level rather than about the J′ value of (or minimum energy gap relative to) the parent level.

Spectroscopic study of Λ-doublet, spin, and rotational relaxation in the NH(A 3Π,v=0) state

Electronically excited NH(A 3Π) radicals in single N′, J′, e/f states were investigated by pumping on isolated NH(A←X) lines of the (0,0) band. Collision-induced transitions among the different Λ-doublet, spin, and rotational states were monitored by fluorescence spectra. In collisions with NH3, a propensity for conservation of spin is observed. In the original spin unit, rotational relaxation occurs preferably to the neighboring rotational levels. The efficiency of spin-unit changes decrease with increasing ΔΩ. For NH3 collisions inducing a fine-structure change, the rotational distribution is found to be thermal and no memory of the original rotational level is left. In collisions with Ar, spin is not conserved. Generally, relaxation into states of the same Λ-doublet component occurs with approximately the same probability as into the other component. Relaxation processes induced by Ar are less efficient than those caused by collisions with NH3.

Collisions and rotational spectroscopy

Motivated by Oka's early work on molecular collisions and especially collision-induced rotational transitions, this paper addresses progress in these areas. Particular attention is paid to recent extensions to lower energy collisions of astrophysical significance and to questions about the relation between experimental observables and the more fundamental molecular interactions.

Spectral line shape arising from collisional interference between electric-dipole-allowed and collision-induced transitions.

A theory is developed to describe the spectral line shape due to interference between electric-dipole-allowed and collision-induced transitions in pure rotational molecular spectra. Motivation was provided by experimental data available for HD-inert gas systems. This theory is based on a master-equation approach to induced spectra employed by Alber and Cooper [Phys. Rev. A 33, 3084 (1986)]. The active molecule is considered to be immersed in a bath of perturbers. An expression for the absorption coefficient is obtained within the binary collision approximation that contains terms due to allowed, induced, and interference contributions. Effects due to m mixing, J mixing, and successive collisions are included. Low-order approximations of the theory eventually reduce to results of earlier efforts, namely, the pioneering description of collisional interference by Herman, Tipping, and Poll [Phys. Rev A 20, 2006 (1979)] and refinements to it through consideration of rotational level mixing. The principal attribute of this approach is the treatment of allowed and collision-induced transitions in a consistent manner.

Systematic study of helium-induced L shell ionization cross sections

An experimental data set consisting of about 500 published cross section values has been applied to test the performance of various theoretical approximations for the description of the helium-induced L-subshell ionization of heavy elements ( Z 2 = 66−92). The large discrepancy observed previously between the L 2-subshell data and the predictions of the ECPSSR model has been greatly reduced including the effect of the collision-induced intrashell transitions. Further improvements in the description of the data, particularly for the L 3 Subshell, have been achieved modifying the binding energy correction of the ECPSSR theory according to the recent suggestion by Vigilante et al. The use of more realistic electronic wave functions than those applied in ECPSSR is discussed as a possible way to remove the remaining discrepancies.

On the rotational angular momentum polarization in N+2–He. Classical trajectory and hard-ellipsoid model calculations

Classical trajectory and hard-ellipsoid methods are used to investigate collision-induced rotational alignment effects in N+2–He. Classical total, mf-resolved, and tensor cross sections for collision-induced rotational transitions are presented. Comparison of classical trajectory and quantum closed-coupled results show that total rotational inelastic cross sections are in good agreement, while mf-resolved and tensor cross sections agree only semiquantitatively. Velocity-averaged alignment parameters for N+2 ions drifting in a helium buffer gas are computed using a hard-ellipsoid model and a semiempirical two-dimensional velocity distribution. The alignment parameters are found to be smaller than the experimental values but lie in the range of the parameters obtained from a former quantum closed-coupled analysis.

Highly state-selective collisional energy transfer between 1Δ g and 1Σ +g Rydberg states in K 2

Collisional energy transfer between 1Δ g and 1Σ + g Rydberg states of K 2 has been studied by optical-optical double resonance (OODR) resolved fluorescence spectroscopy. The collision-induced probability for transition from a 1Δ g state to an adjacent 1Σ + g state of K 2 depends much more strongly on Δ J than on Δ E in each case studied. Δ J = 0 collision transfer is more favorable than Δ J = ±2, … for 1Δ g odd Je levels. For 1Δ g even Jf levels, Δ J = ± 1 transfers are most favorable. The measurements of intensities of 1Δ g, 1Σ + g ← B 1Π u, υ′ = 2, J′ = 14e transitions and fluorescence intensities show that the collision-induced transition probability of these 1Δ g- 1Σ + g systems is quite large.

Energy-gap dependence for inelastic collision-induced electronic transitions in N2+

An optical-optical double resonance procedure is used to compare the relative populations of e/f spin components of the $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$(v=7) rotational manifold resulting from the collision-induced electronic transitions in ${\mathrm{N}}_{2}^{+}$ for the cases of large and small energy gaps. Rotational levels of the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{ui}}$(v=3 or 4) vibrational state of ${\mathrm{N}}_{2}^{+}$ are selectively populated with a pump laser. Transitions to rotational levels of the $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$(v=7) level due to collisions with bath gas helium atoms at room temperature are monitored by scanning the lines of the B-X(5,7) band with a second, probe laser. The relaxation pathways A(v=3)\ensuremath{\rightarrow}X(v=7) and A(v=4)\ensuremath{\rightarrow}X(v=7) traverse energy gaps (\ensuremath{\Delta}E) of \ensuremath{\sim}0 and \ensuremath{\sim}1760 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. Perturbation of the rotational levels of the ${\mathit{B}}^{2}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$(v=5) state by the ${\mathit{A}}^{2}$${\mathrm{\ensuremath{\Pi}}}_{\mathrm{u}1/2}$(v=17) manifold allows for the e/f, spin-rotation splitting of the lines of the B-X(5,7) probe laser scan to be resolved. The effects of these perturbations on the relative intensities of the B-X(5,7) lines have been taken into account. The intensity patterns for collision-induced transitions across the two widely different energy gaps are nearly identical, indicating that the propensities for this case have an insignificant energy-gap dependence. These detailed propensities from selectively excited rotational levels of the A(v=4 or 3) level to the individual, nearly degenerate rotational levels of the X(v=7) manifold are compared with theoretical models for inelastic scattering between $^{2}\mathrm{\ensuremath{\Pi}}$ and $^{2}\mathrm{\ensuremath{\Sigma}}$ electronic states.

ChemInform Abstract: Investigation of Collision‐Induced Transitions for CH3Br by Radiofrequency‐Infrared Double Resonance.

ChemInformVolume 21, Issue 42 Physical Organic Chemistry ChemInform Abstract: Investigation of Collision-Induced Transitions for CH3Br by Radiofrequency-Infrared Double Resonance. F. SCAPPINI, F. SCAPPINI Ist. Spettrosc. Mol., CNR, I-40126 Bologna, ItalySearch for more papers by this authorG. DI LONARDO, G. DI LONARDO Ist. Spettrosc. Mol., CNR, I-40126 Bologna, ItalySearch for more papers by this authorH. + MAEDER, H. + MAEDER Ist. Spettrosc. Mol., CNR, I-40126 Bologna, ItalySearch for more papers by this author F. SCAPPINI, F. SCAPPINI Ist. Spettrosc. Mol., CNR, I-40126 Bologna, ItalySearch for more papers by this authorG. DI LONARDO, G. DI LONARDO Ist. Spettrosc. Mol., CNR, I-40126 Bologna, ItalySearch for more papers by this authorH. + MAEDER, H. + MAEDER Ist. Spettrosc. Mol., CNR, I-40126 Bologna, ItalySearch for more papers by this author First published: October 16, 1990 https://doi.org/10.1002/chin.199042042AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue42October 16, 1990 RelatedInformation

Investigation of collision-induced transitions for CH3Br by radio-frequency–infrared double resonance

The highly sensitive method of radio-frequency–infrared double resonance spectroscopy inside the cavity of a CO2 laser has been applied to the study of collision-induced transitions of CH379Br and CH381Br in the four level scheme. Various types of radio-frequency transitions corresponding to A1–A2 and pure quadropole resonances were observed as a result of collision population transfer between different rotational levels. Thus, collision-induced dips have been measured in the pure gas and in mixtures with polar carbonyl sulfide (OCS) and nonpolar (He, Ar) foreign gases. Clamping experiments were also done, using a second radiation in a six level triple resonance configuration to investigate collision selection rules. The intensity ratios of the collision-induced dips to the corresponding three level double resonance signals were measured for a number of transitions in the ground and in excited vibrational states. Collision selection rules in the pure gas and in mixtures with polar and nonpolar foreign gases are discussed.

RKR Potential, FCF and Mixing Coefficients of the A1 Σu + and b3Πu States of Na2

Abstract A study of the spin-orbit interaction between the A1Σu + and b3Πu states of Na2 , based on the collision-induced transitions (2) 1Σ g →A 1 Σu + recorded with a high resolution Fourier transform spectrometer, had led to the determination of the deperturbed constants of the A1Σu + and b3Πu states [1]. From these constants the Rydberg-Klein-Rees (RKR) potential curves for the A1Σu + (0≦v≦ 15) and b3Πu (0 ≦ v ≦ 25) states and the Franck-Condon factors (FCF) within the range of vibrational levels involved in the interaction of these two states are calculated, together with the mixing coefficients for the pair (A1Σu +)v=4 - (b3Πu)v=10.

Electronic transitions from the A 2Πu i(v=3) level of N+2 induced by inelastic collisions with helium atoms

An optical–optical double-resonance technique utilizing two pulsed lasers is used to study collision-induced electronic transitions from the N+2 A 2Πui(v=3) level by helium. Collisional deactivation paths are determined by this technique and found to be between this level and the X 2Σ+g (v=7 and 6) levels. The same propensity for ΔJ≊0 occurs for both of these paths in spite of an electronic energy gap size of approximately 0 cm−1 between the A(v=3) and X(v=7) levels and a large gap size of about 1950 cm−1 between the A(v=3) and X(v=6) levels. The electronic quenching rate from A(v=3) to X(v=7) is found to be only about three times larger than that to the X(v=6) level. We use this branching ratio in an electronic relaxation model to determine the collisional quenching rates between the A(v=3) and X(v=7 and 6) levels. These state specific rates are determined by fitting the model to observed radiative decay curves from the A(v=3) level obtained at various helium pressures. There is excellent agreement between the analytical and observed decay curves. The relatively efficient nature of the collision-induced electronic transition over the large energy gap is somewhat surprising in view of the fact that the nitrogen ions and helium atoms must remove most of this energy as translational kinetic energy. We have also revised our previous rate constants from the A 2Πui(v=4) level for 14N+2 and 15N+2.

Two-dimensional experiments with collision-induced transfer of populations in microwave Fourier transform spectroscopy

The two-dimensional (2D) technique in microwave Fourier transform (MWFT) spectroscopy has been extended to four-level double resonance. By applying a three-pulse sequence with a particularly selected phase cycle, it was possible to correlate the spectra excited by pump and signal radiation in two dimensions. Cross peaks in such 2D spectra appeared only when collision-induced transitions took place between the levels of the irradiated transitions. Also the dynamics of such collision processes could be investigated. The power of the method is greatly enhanced by the high sensitivity of the MWFT technique. Experiments were performed on HC15N in order to investigate the collision-induced transitions between different pairs of l-type doublets. The one-dimensional analog of this 2D method was tested on acetaldehyde.

Nuclear spin relaxation by intermolecular magnetic dipole coupling in the gas phase. 129Xe in oxygen

The nuclear spin relaxation times (T1) of 129Xe in xenon–O2 gas mixtures have been measured as a function of temperature and density at different magnetic fields. This system is used to characterize the intermolecular dipolar relaxation of nuclear spins in the gas phase. An empirical Boltzmann-averaged collision cross section associated with the collision-induced transitions between 129Xe nuclear spin states is obtained as a function of temperature.

An exponential two-state model for collisional excitation via a continuum

The authors consider a new case of collision-induced transitions between two discrete (quasistationary) states embedded in a continuum. The energy difference ( delta =E1-E2) between the two states is assumed to remain constant throughout the collision whereas the difference between their individual decay widths ( Gamma 1- Gamma 2) and their imaginary couplings (iV1,2) via the continuum have a common exponential dependence: (E1-i Gamma 1/2)-(E2-i Gamma 2/2)= delta -ig exp(- eta R) iV1,2=iF1,2 exp(- eta R). Transition probabilities are determined in closed analytical form for this problem.

Rotational transitions in compound muonic molecules.

Cross sections and rate constants are calculated for collision-induced rotational transitions of compound muonic molecules, which determine relaxation rates important to muon-catalyzed fusion processes. The interaction potential is derived from the ${\mathrm{H}}_{2}$-${\mathrm{H}}_{2}$ potential, and the R-matrix propagation method is used to solve the set of coupled collisional equations for the systems ${\mathrm{D}}_{2}$+[(dt\ensuremath{\mu})dee], ${\mathrm{D}}_{2}$+[(dd\ensuremath{\mu})dee], ${\mathrm{T}}_{2}$+[(dt\ensuremath{\mu})dee], ${\mathrm{D}}_{2}$+[(dt\ensuremath{\mu})tee], and ${\mathrm{H}}_{2}$+[(dd\ensuremath{\mu})pee]. Calculations are also done on ${\mathrm{H}}_{2}$+HD and ${\mathrm{D}}_{2}$+HD for comparison with previous work.

A time-resolved study of rotational and vibrational excitation and relaxation in the<sup>13</sup>CH<inf>3</inf>F optically pumped far-infrared laser

The rotational and vibrational energy transfer processes of the13CH 3 F optically pumped far-infrared (OPFIR) laser have been studied in a time-resolved experiment. The experiment uses a tunable millimeter and submillimeter spectroscopic technique as a diagnostic probe. Included are observations of the fast \Delta J processes that closely connect other J states within K = 3 to the directly pumped J = 5 level, a vibrational swapping mechanism that transfers excitation from K = 3 to other K states, vibrational relaxation due to both wall collisions and molecule-molecule collisions, the nonunity probability of vibrational deactivation in a wall collision, and pump saturation and hole burning effects due to the CO 2 pump laser. All of these observations are accounted for in the context of a numerical simulation. This results in a complete map of all of the collision-induced rotation-vibration transitions of importance to this basic OPFIR system including quantitative cross sections for the relevant processes.

Collision-induced two-photon forbidden 3s21S-3snp1P transitions and collisional enhancement of 3s21S-3sns1S transitions in Mg I

The collision-induced two-photon forbidden 3s2 1S-3snp 1P transitions (n>11) and collisional enhancement of 3s2 1S-3sns 1S transitions (n>8) of Mg I have been observed and studied as functions of Mg vapour pressure and perturbing gas (Ar) pressure by using two-photon resonant ionisation and thermionic diode techniques. The occurrence of forbidden transitions and collisional enhancement of 3s2 1S-3sns 1S transitions is qualitatively attributed to the formation of a Rydberg Mg2 excimer and a MgxAry Fermi complex. Quantitatively, the above phenomena are interpreted as collisional mixing of high Rydberg states of Mg. The mixing coefficients are estimated by using perturbation theory and the semiclassical (JWKB) approximation. The theoretical estimation is in good agreement with the experimental result.

Collisional broadening of rotation–vibration lines for asymmetric-top molecules. II. H2O diode laser measurements in the 400–900 K range; calculations in the 300–2000 K range

Numerous comparisons between predictions of the model presented in part I of this paper and experimental H2O infrared linewidths are presented. It is shown that our model, contrary to those used up to now, gives accurate results for H2O room-temperature line broadening by O2 and Ar, and for high rotational quantum-number lines by N2. First accurate experimental widths and intensities of some H2O ν2-band lines in the 400–900 K temperature range are also presented. Detailed analysis of the data demonstrates the great influence of a ‘‘resonance overtaking’’ mechanism. The latter results from the modifications of both the perturber rovibrational population distribution and kinetic energy with temperature; it strongly enhances the contributions of the collision-induced rotational transitions involving significant energy jumps. This mechanism is well accounted for by our model and quantitatively explains the unusually slow decrease of some linewidths with temperature.

Collision cross sections for ammonia in its ground vibrational manifold

Rates for inelastic collision-induced transitions between rotation–inversion states are measured for the NH3 molecule colliding with He atoms and with N2 molecules. An infrared–microwave double resonance method is used to determine differences within pairs of collisional transition rates connecting different inversion pairs in the ground (ν2=0) vibrational manifold of NH3. The measured rates are analyzed for parity, and J and k selection rules. The results are consistent with Oka’s saturation experiments on NH3, but in some respects inconsistent with theoretical calculations of collisional transition rates.