Specific Rotational Levels Research Articles

State-resolved study of collisional energy transfer between A 2Π v=7 and X 2Σ+ v=11 rotational levels of CN

Collisional transfer from the A 2Π state of CN has been studied with initial and final state resolution by an optical–optical double resonance technique. Specific rotational levels in the v=7 vibrational manifold of the A state of CN in a flow of several Torr of argon are prepared by pulsed laser excitation in the A–X (7,2) band. After a short time delay, a second laser probes the populations of quantum levels in this vibrational manifold and in the nearly isoenergetic v=11 manifold of the X 2Σ+ state by fluorescence excitation in the overlapped B–A (8,7) and B–X (8,11) bands. The interelectronic A→X transfer rate is found to be comparable to that for purely rotational collisional transitions within the A state for all incident levels studied, regardless of whether or not they possess significant X state character, because of isolated molecule non-Born–Oppenheimer mixing. Reflecting the near homonuclear character of the CN–Ar interaction potentials, the final X state populations exhibited a significant even–odd alternation as a function of the final rotational angular momentum quantum number. These populations could be adequately fit by the sudden scaling relationship for Π→Σ cross sections derived by Alexander and Corey [J. Chem. Phys. 84, 100 (1986)].

Interference effects in rotational-state-resolved collisional energy transfer between the A 2Π and X 2Σ + states of CN

An optical-optical double resonance technique has been used to study transfer between specific quantum levels in the A 2Π and X 2Σ 2 states of the CN radical in collisions with argon. Specific rotational levels within the v = 7 A state vibrational level are prepared by laser excitation in the A-X (7,2) band, while collisionally populated levels in this vibrational level and the nearly isoenergetic v =11 level in the X state are probed by laser fluorescence excitation in the interleaved B-A (8,7) and B-X (8,11) bands. The collisional transfer rate from the relatively unperturbed N = 6 F 1 λ-doublets to the X state is comparable to or faster than that for pure A → A inelastic transitions. A significant alternation in the population of even versus odd N levels in the X state is observed, related to the near-homonuclear character of the CN-Ar interaction potentials.

Rotational-level-dependent quenching of A 2Σ+ OH and OD

Rate constants kQ for collisional quenching of A 2Σ+, v′=0, OH and OD have been measured for specific rotational levels N′ of the radical and a wide variety of collision partners. Through measurements of the time-dependent laser-induced fluorescence in a low pressure discharge flow at room temperature, we observe a decrease in kQ with increasing rotational quantum number for most quenchers. The internal levels of the collision pairs appear unimportant from experiments involving deuterium substitution. A comparison of rotationless rates for different colliders [kQ(N=0)] with calculations based on collision complex formation indicate that attractive forces play a role in the quenching process.

Measurement of the fragment kinetic energy distribution in laser photopredissociation of N 2O +

Using an electrostatic energy amlyzer together with a fast-ion-beam laser spectrometer, we have measured the kinetic energy spectrum of NO + ions produced in the slow predissociation of N 2O + prepared in specific vibrational and rotational levels of the à excited electronic state. For either the à 2 (100) or à 2Π(b) (110) states the resulting distribution is remarkably narrow, with almost all of the NO + ions produced in the ν″ = 4 vibrational level.

Radiative lifetimes and quenching rate coefficients for directly excited rotational levels of OH (A 2Σ+,v′=0)

A narrow bandwidth pulsed dye laser has been used to excite OH X 2Πi radicals to the A 2Σ+ state by pumping in the (0,0) vibrational band around 308 nm. The radiative lifetimes of specific (K′,J′) rotational levels in v′ = 0 were measured at low pressures ⩽1 mTorr which gave a mean lifetime τ0=0.721±0.009 μs (2σ). Electronic quenching rate constants for important atmospheric species N2, O2, H2O, and also for H2 were measured for a range of initially excited rotational levels. A strong dependence of this rate constant on the initially excited level was found for N2, and less markedly for O2, with the rate constant tending to increase for the lowest rotational levels K′⩽3. The implications of these measurements of radiative and quenching rates for state selected rotational levels of OH A, v′=0 to the laser-induced fluorescence detection of atmospheric OH are discussed.

OH rotational temperature from two-line laser-excited fluorescence

A two-line laser-excited fluorescence technique has been developed to measure the rotational temperature of the OH molecule. This technique eliminates problems encountered in the application of other laser fluorescence methods for measuring the OH temperature in combustion environments, such as fluorescence trapping, nonequilibrium excited state population, spectral bandwidth sensitivity, and quenching. The technique consists of exciting a specific rotational level of the OH molecule in the A(2)Sigma (upsilon' = 0) excited state from two different rotational levels in the X(2)Pi(upsilon'' = 0) ground state using a tunable dye laser and monitoring the broadband fluorescence. An example of the implementation of this technique in an atmospheric pressure methane-air flat flame is included. The possible application of this technique in turbulent combustion is also evaluated.

Multiphoton ionization of molecules in selectively excited rovibrational states

Reported are observations of multiphoton ionization of a molecule (NO) due to sequential excitation by a tunable infrared laser and a tunable dye laser. These double-resonance experiments yield multiphoton ionization spectra of specific rotational levels, selectively populated by direct IR absorption. This technique simplifies complex multiphoton ionization spectra and offers a means of sensitively and selectively detecting IR absorption of molecules.

Rotational population transfer in HF

Population transfer in HF by both collisional and radiative processes was measured by ir-laser double-resonance experiments. In three kinds of experiments populations pumped to a specific rotational level were followed: out of the pumped level, to the levels above, and to the levels below. The rates of loss from the pumped level and transfer to the lower levels were strongly affected by lasing between rotational levels. Data for gain and laser intensity were incorporated into a kinetic rate model to evaluate transfer rates.

MIT patent muddies chemical uses for lasers

One of the most exciting areas of contemporary photochemistry is the use of infrared lasers as the energy source for photochemical reactions. Because they can be tuned to excite specific vibrational and rotational energy levels in particular molecules, these lasers are being used to increase the yield of particular products, often ones that are not favored under ordinary reaction conditions. The technique is still largely experimental, but it offers significant promise in areas such as isotope separation, uranium enrichment, and petroleum cracking and refining. But new complication may have appeared in the development of this technology. Massachusetts Institute of Technology has received patent that claims to cover essentially all applications of infrared lasers to chemical or biological reactions. MIT is not seeking the exclusive right to develop this technology, but rather the right to collect licensing fee from anyone with commercial application for the technique. The MIT patent covers a metho...

Laser induced fluorescence from NH2(2A1). State selected radiative lifetimes and collisional de-excitation rates

Time resolved fluorescence from the first excited (2A1) state of NH2 has been observed following excitation of the radical in its ground state by means of a pulsed tunable dye laser. Specific rotational levels within a number of vibronic states were populated, decay rates measured as a function of total pressure for a variety of added gases, and zero pressure lifetimes and collisional de-excitation rates evaluated. Measured zero pressure lifetimes are good approximations to the vibrational state radiative lifetimes, typically 10 μsec for the (0, 9, 0) state. Collisional de-excitation rate constants were measured as 1.0×10−9 cm3 molecule−1⋅sec−1 for NH3, independent of vibronic state, and for the Σ (0, 9, 0) level were found for other gases in the ratio NH3:CO:H2:N2:CH4:Ar:He=1.0:0.47:0.46:0. 40:0.30:0.152:0.145. Using excitation by a tunable cw dye laser, steady state spectra of NH2 have been obtained and collisional energy transfer observed within the (2A1) excited electronic state of NH2. Transfer was observed both within the initially populated vibronic state and to other such states within the same overall vibrational level. The symmetric or antisymmetric character of the rotational level remained unchanged in collision, i.e., only a↔a and s↔s transfer occurred.

Energy transfer processes in monochromatically excited iodine. IX. Classical trajectory and semiclassical calculations of vibrationally and rotationally inelastic cross sections

Classical trajectory calculations were carried out on the system in which I2 in the 3Π0μ+ excited electronic state and certain specific vibrational and rotational levels interact with the inert gases as collision partners. The trajectory results duplicated the effective cross sections for vibrational and rotational energy transfer into specific channels, as well as the mass dependence of the total cross sections for vibrational and rotational energy transfer when a correction was made for quenching. A semiclassical model provided less accurate values for the vibrational deactivation probabilities, but at a considerable saving of computation time.

Singlet-triplet resonance interaction of the [formula omitted] and [formula omitted] states of propynal

The 0-0 absorption band of the A ̃ 1A″- X ̃ 1A′ system of propynal, C 2H·CHO, shows Zeeman effects in individual lines of the rotational structure in a magnetic field of 7.8 kG. These effects are attributed to a near-resonance interaction of specific rotational levels of A ̃ 1A″ with levels of an excited vibrational state of a ̃ 3A″ . The analysis indicates that the zeroth vibrational state of A ̃ 1A″ interacts principally with a nontotally-symmetric vibrational state of 3 A″, possibly ν 6( a′) + ν 7( a′) + ν 10( a″), by vibronic spin-orbit coupling.

Energy Transfer in Monochromatically-Excited Hydrogen (B 1Σ u+). I. Excitation Processes, Electronic Quenching, and Vibrational Energy Transfer

Excitation of HD molecules into specific vibrational and rotational levels of the B 1Σ u+ electronic state by absorption of the 1048 and 1066 Å argon resonance lines has been studied under various experimental conditions by observing the steady-state fluorescence spectra of the (B 1Σ u+→ X 1Σg+) Lyman bands. Strong excitation of the (v′ = 3, J′= 2), (v′ = 5 , J′=2), and (v′ = 6 , J′ = 5) levels was achieved. The HD (B 1Σ u+ → X 1Σg+) electronic fluorescence from v′=3 was found to be quenched by HD, 3He, 4He, and Ne with effective collision cross sections of 79, 8.8, 9.9, and 3.5 Å2 at 297°K. Vibrational energy transfer processes HD(B 1Σ u+ ; v′=3, J′) + M→ HD(B 1Σ u+; v′=2, J′ + Δ J′ ) + M+ Δ Ewere studied for the collision partners 3He, 4He, and Ne. Large effective cross sections of 0.89, 0.76, and 0.81 Å2 were found at 303°K. Rotationally-resolved spectra for Ne show that vibrational energy is transferred into both rotation and translation. All energetically accessible values of ΔJ′ were observed. On the average one-half of the vibrational energy was transferred into rotation.