A. F. Vilesov

High-resolution infrared spectroscopy of SF6 embedded in He clusters

The infrared absorption spectrum of SF6 embedded in large liquid HeN clusters was studied in the range around 946.3 cm−1 using a cw diode laser. A sharp peak (fwhm < 300 MHz) accompanied by two weaker satellite bands of larger halfwidth (≈ 1 GHz) were recorded for different mean He cluster sizes (〈N0〉 = 1000–5000). The satellite bands are size dependent and can be fit by assuming that the SF6 molecules with several rigidly attached He atoms rotate freely within the He cluster. Phonon satellites involving several different types of possible vibrations are also considered as a possible explanation.

Vibrational‐state‐to‐state collision‐induced intramolecular energy transfer N2(A 3Σu+, v‘→B 3Πg, v’)

Absolute cross sections for collision‐induced intramolecular energy transfer from the metastable A 3Σu+ state into the radiating B 3Πg state of N2 have been measured for the first time under single‐collision conditions, using a thermal energy molecular beam of N2(A). The collision partners studied were the five rare gases, H2, N2, NO, and O2. The product vibrational levels (B, v’=4–10) were separated using spectrally resolved detection by means of filters as in our earlier related work [R. Bachmann, X. Li, Ch. Ottinger, and A. F. Vilesov, J. Chem. Phys. 96, 5151 (1992)]. In addition, in the present study the contributing reactant state vibrational levels (A,v‘) were labeled, using optical pumping by a specially developed broad‐band (∼1 nm) pulsed tunable dye laser. A depletion of up to 30% of a given v‘ level could be achieved, about one‐half of the theoretical maximum, at a pump pulse energy of 4 mJ. This quantity was also measured directly using a second synchronized probe laser. Pumping on a particular...

Molecular‐beam study of the collisional intramolecular coupling of N2(B 3Πg) with the N2(A 3Σ+u) and N2(W 3Δu) states

Collision‐induced intramolecular energy transfer between N2 triplet states has been investigated for the first time under single‐collision conditions. A beam containing N2 molecules in the long‐lived N2(A 3Σ+u) and N2(W 3Δu) states interacted with target particles (H2, N2, NO, and all rare gases) either in a collision cell or in a secondary, pulsed molecular beam. From the collision region N2(B 3Πg) emission was observed with a linear dependence on the target‐gas density. It is due to collision‐induced intramolecular energy transfer (‘‘collisional coupling’’) N2(W→B) and N2(A→B). These two contributions were differentiated by means of data taken at different distances from the beam source, using the known radiative rate decay of N2(W). The spectra (40 A full width at half maximum) show clearly the importance of energy resonance between reactant and product vibrational levels, with an exponentially decreasing dependence of the cross section on the energy mismatch. Relative cross sections were obtained for ...

Isotopic study of the intermolecular versus intramolecular energy transfer in the N2(W,A)+N2(X) reactions

The energy transfer from the long‐lived states N2(W 3Δu, A 3Σ+u) to the radiating state N2(B 3Πg) in collisions with N2(X 1Σ+g) was studied under single collision conditions, employing a molecular beam/target gas cell arrangement. By means of using the isotopic species 14N2 in the metastable reactant beam and 15N2 in the target gas cell it was possible to differentiate between the intramolecular and the intermolecular energy transfer mechanisms, on the basis of well‐resolved N2(B) product emission spectra. The overall contributions of the two reaction channels were found to be comparable, but they differ greatly in the vibrational product distributions. The intermolecular process populates preferentially the low vibrational levels of 15N2(B,v). The intramolecular process is most efficient for those 14N2(B,v) levels which are in close energy resonance with N2(A or W) vibrational levels.

Intramolecular collisional transfer in NO (a 4Π→B 2Π, b 4Σ−): Gateway‐type, resonant versus direct, nonresonant mechanisms

A beam containing NO in the long‐lived a 4Π state was allowed to interact with target particles in a collision cell. Intramolecular collision‐induced transitions a 4Π→B 2Π and a 4Π→b 4Σ− were observed via the subsequent emissions in the β bands (B 2Π→X 2Π) and the Ogawa bands (b 4Σ−→a 4Π), respectively. In the ultraviolet part of the spectrum long β band progressions originating from the B‐state vibrational levels v=0 and 3 were observed. In each band only a few lines appear, which were assigned to transitions from the rotational levels 2Π3/2(10.5) in v=0 and 2Π1/2(17.5) in v=3. These particular B 2Π levels are perturbed by specific levels of the a 4Π state, serving as so‐called gateways to allow the otherwise spin‐forbidden a 4Π→B 2Π collision‐induced transition. An external magnetic field has a strong effect on the collision‐induced emission from the NO(B, v=0) level. With paramagnetic target gases, direct spin‐changing NO(a→B) collisional transfer was also observed in addition to the gateway transition...

Lifetime measurements on perturbed levels of NO(B 2Π) and precise term energy of the NO(a 4Π) state

Using a pulsed near‐UV dye laser, the laser‐induced fluorescence (LIF) excitation spectrum of the NO(B 2Π←X 2Π) transition was measured in the (0,9) band on vibrationally hot NO in a molecular beam. Lifetime measurements were made for some of the B,v=0 and v=3 rotational/fine structure levels, including one which was recently shown to exhibit very specific kinetic effects due to a perturbation by the NO(a 4Π) state [Ch. Ottinger and A. F. Vilesov, J. Chem. Phys. 100, 1805 (1994)]. This perturbation manifested itself in the present work by a significantly longer lifetime of the 2Π3/2(10.5) level relative to other B,0 levels. The same effect was observed for the B,v=3 2Π1/2(17.5) level. The S/O interaction matrix element is estimated to be very small (≂10−2–10−3 cm−1). The perturbed level pairs must therefore be in very close, accidental coincidence. This was used for a precise determination of the term energy of the a 4Π state as T0(a 4Π)=38 266.74±0.03 cm−1.

A new band system of nitrogen: Observation of the N2(G 3Δg→W 3Δu) transition

The first experimental observation of the N2(G 3Δg→W 3Δu) transition is reported on here. The emission forms part of the spectrum of the so‐called N2 beam afterglow, a spontaneous luminescence emitted by a molecular beam of N2 issuing from an intense d.c. discharge. Using a high performance charge‐coupled device (CCD) optical multichannel detector, 18 bands of a new band system were observed with 2 A full width at half‐maximum (FWHM) resolution in the 350–650 nm region. Three well‐resolved v‘ progressions were analyzed. From a comparison with the known vibrational spacings in the N2(G) and N2(W) states, they could be assigned unambiguously to the N2(G→W) transition. This observation allows the energy of the N2(G) state to be determined as Te=89 505 cm−1 or 11.10 eV, thereby also fixing the location of the previously observed H(3Φu) state at Te=107 328 cm−1 or 13.31 eV.

Collision‐induced transitions from N2(A’ 5Σ+g) to N2(B 3Πg) via the gateway mechanism

A beam containing N2 in long‐lived states was allowed to interact with target particles in a collision cell. Intramolecular transitions A→B and W→B are induced, such as were studied earlier by us by means of the subsequent B→A emission [R. Bachmann, X. Li, Ch. Ottinger, and A. F. Vilesov, J. Chem. Phys. 96, 5151 (1992)]. In the present work the product emission was observed under high resolution (1 A FWHM). Most of the B→A bands show the typical quasithermal rotational contours. However, in the emissions from the B state vibrational level v=10 sharp superimposed features were observed. They were assigned to transitions from the rotation/fine structure/Λ sublevel 3Πe2(12). This particular level is perturbed by the A’ 5Σ+g state, serving as a so‐called gateway to allow the otherwise spin‐forbidden 5Σ+g→3Πg collision‐induced transition. According to this mechanism, the collisions scramble only the levels within the A’ state, while the A’→B transition occurs spontaneously through S/O coupling. A similar, less...

Laser spectroscopy of perturbed levels in N2(B 3Πg,v=10) and the first experimental determination of the N2(A’ 5Σ+g) term energy

Using both a pulsed and a narrow‐band cw dye laser, laser‐induced fluorescence excitation spectra of N2(B,v=10) were observed on the N2(B 3Πg←A 3Σ+u) transition from the metastable N2(A) component of a molecular beam. Lifetime measurements were made for some of the B,v=10 rotational/fine structure levels, including one which was recently shown to exhibit very specific kinetic effects due to a perturbation by the N2(A’ 5Σ+g) state [Ch. Ottinger, L. G. Smirnova and A. F. Vilesov, J. Chem. Phys. 100, 4848 (1994)]. The perturbation manifested itself in the present work by a significantly longer lifetime of the 3Πe2(12) level relative to other B,10 levels, as well as by line shifts. The S/O interaction matrix element is obtained to be 0.35 cm−1. From the required very close accidental coincidence between well‐identified levels in the B and A’ states the term energy of the latter could be precisely determined as Te(A’ 5Σ+g)=75 990.0 cm−1. This is the first measurement of this value, and the result is ≂440 cm−1 ...

High-resolution study of collision-induced transitions from N2(A 3Σ+u, ν=10) to N2(B 3Πg, ν=2)

Abstract The collision-induced intramolecular energy transfer N 2 (A 3 Σ + u )+M→N 2 (B 3 Π g )+M was studied under molecular beam conditions using Xe and N 2 as the collision partners. Specifically, the near-resonant transfer between the levels A, ν″=10→B, ν′=2 was investigated by means of the product emission, isolated by an interference filter. The rotational and fine structure levels of the A, ν″=10 reactant state were labeled by selective narrow-band laser pumping. The laser was scanned over the whole B, 12←A, 10 band and the subsequent decrease in the collision-induced emission B, 2→A, 0 was recorded. Repeating the scan, the laser-induced fluorescence intensity was recorded. These spectra are similar, showing that there is no significant dependence of the cross section for collision-induced energy transfer on the rotational or fine structure state of the reactant N 2 (A, ν″=10) molecules.