Inverse Electronic Relaxation Research Articles

Infrared multiphoton dissociation of tetramethylsilane: formation of electronically excited trimethylsilyl radical

The infrared multiphoton excitation of tetramethylsilane and chloromethyltrimethylsilane molecules, under collision-free and collisional conditions, with a tunable CO 2 laser resulted in an intense ultraviolet—visible luminescence. The spectral analysis and the time profile of this luminescence has been studied, in order to provide a reasonable mechanism of its formation. Lifetime measurements of this emission were performed at various alkylsilane pressures, and its radiative lifetime was found to be 365 ± 10 ns. The quenching rates of this luminescence by several colliders were also determined from lifetime measurements. The proposed mechanism leading to the production of this chemiluminescence involves the formation of electronically excited trimethylsilyl radicals (1 2E state) via an inverse electronic relaxation process.

Multiphoton absorption and luminescence of chromyl chloride

Infrared multiphoton absorption (MPA) of chromyl chloride (CrO 2Cl 2) and luminescence, due to inverse electronic relaxation (IER), were studied in the gas phase at pressures of 6.7 to 133 Pa (0.050 to 1.00 Torr) with CO 2 laser pulses of 10 and 60 ns duration. A dependence of MPA measured photoacoustically, on pulse length, was observed at 6.7 Pa but not at 67 Pa, with a factor of 2.5 greater absorption for the shorter laser pulse at the lower pressure. A much larger dependence of luminescence intensity on laser pulse duration was observed at 6.7 Pa with a factor of 10 to 40 increase in luminescence signal for the shorter laser pulse. The origin of the intensity dependences in the MPA and luminescence experiments is discussed and it is shown that the two dependences are not completely related.

Inverse electronic relaxation in the case of multiphoton excitation of molecules by infrared laser radiation

The first systematic account is presented of the current status of the problem of inverse electronic relaxation under the conditions of multiphoton excitation with infrared laser radiation, which converts a strong vibrational excitation of a molecule into an electronic excitation and gives rise to ultraviolet or visible radiation. This review deals with the fundamental principles of the appearance of inverse electronic relaxation in isolated molecules. An analysis is made of the characteristics of visible radiation generated by this relaxation process. Current ideas are used in a detailed investigation of those characteristics of infrared multiphoton excitation which determine inverse electronic relaxation. An account is given of the recently developed experimental methods for attribution of the observed radiation to a specific particle (molecular fragment). These methods include time-of-flight spectroscopy and separation of successive stages of the emission process in accordance with the laser radiation energy density. A critical analysis is made of the numerous experimental observations of collisionless visible radiation resulting from multiphoton excitation by infrared laser radiation. Some of the most thoroughly investigated molecules are used to consider the cases of inverse electronic relaxation resulting from multiphoton excitation of specific molecules and radicals, particularly SO2 and OsO4. Apart from inverse electronic relaxation, attention is given to another nonadiabatic process occurring in the case of infrared multiphoton excitation, which is the detachment of electrons from molecular ions induced by CO2 laser radiation. The review covers the work published up to September 1986.

Obratnaya elektronnaya relaksatsiya pri mnogofotonnom vozbuzhdenii molekul infrakrasnym lazernym izlucheniem

Обратная электронная релаксация при многофотонном возбуждении молекул инфракрасным лазерным излучением, Пурецкий А.А., Тяхт В.В.

Transient Isomerisation and Inverse Electronic Relaxation of Infrared Multiple‐Photon ExcitedPentafluoropyridine

Pentafluoropyridine (PFP) undergoes fast structural isomerisation to fulvenes when irradiated with 9R(16) CO2 laser line in the fluence range 0.5–1.5 J/cm2. The unstable fulvenes slowly decay back to PFP in ms time scale. No detectable permanent dissociation of PFP was observed in the above low fluence experiments. However, by using focused CO2 laser beam for the irradiation of PFP, an emission of light in the visible/near‐UV was observed. The time evolution of the luminescence reveals three peaks at 390, 460 and 500 nm on a broad background emission of 300–680 nm. These bands are assigned to the fluorescence of PFP via inverse electronic relaxation (IER) and C2 Swan bands. On prolonged irradiation of PFP in focused condition a small extent of permanent dissociation was observed with the major products as C2F4, a sooty yellow deposit and another compound presumed to be Dewar PFP.

Study of the visible emission induced by IR multiple-photon excitation of OsO4

The visible luminescence induced in the OsO4 molecule by the action of CO2-laser radiation was studied in a wide range of laser fluences (2–1000 J/cm2). A novel approach consisting in separating successive dissociation and fluence was developed experimentally to study the visible emission induced by multiple-photon excitation (MPE) in an IR-laser field. Three different luminescence stages with definite fluence thresholds were found in MPE of the OsO4 molecule. The luminescence spectra and the velocities of the luminescent species were measured at each stage. A theoretical model is proposed which explains the observed three-stage emission in OsO4 by the following sequence of processes taking place during the IR-laser pulse: inverse electronic relaxation (IER)–multiple-photon dissociation (MPD) of the parent molecule (OsO4–first stage)—IER–MPD of the primary fragment (OsO3-second stage)—IER–MPD of the secondary fragment (OsO2-third stage).

Infrared multiphoton excitation of SO 2 to fluorescent states

SO 2 molecules excited under essentially collision-free conditions by intense (>20 G/Wcm 2) CO 2 laser pulses give off a broad UV—visible luminescence through inverse electronic relaxation. Unlike in larger polyatomics, the infrared multiphoton excitation process is controlled by the laser intensity and not the laser fluence.

Silicon tetrafluoride visible luminescence induced by high-power CO 2 laser irradiation

The visible luminescence induced by a high-power CO 2 laser in SiF 4 occurs within the laser pulse even at pressures as low as 10 −2 Torr. The unirnolecular nature of the process is also supported by pressure-dependence studies of the luminescence intensity and the incubation period. A possible assignment of the transition involving inverse electronic relaxation is discussed.

Inverse electronic relaxation and chromyl chrolide: the final conflict

Inverse electronic relaxation and chromyl chrolide: the final conflict

Fluorescence excitation and emission spectra of polycyclic aromatic hydrocarbons at flame temperatures

Fluorescence excitation and emission spectra of polycyclic aromatic hydrocarbons have been measured in an atmospheric flame at temperatures ≈1200–1500 K. The spectra do not differ greatly from those at lower temperatures (≈400–500 K), indicating little net change in vibration on excitation. Inverse electronic relaxation between S 1 and S 2 states in pyrene is observed.

Observation of direct infrared multiphoton pumping of the triplet manifold of biacetyl

Direct collisionless multiphoton (MP) excitation of the triplet vibronic manifold of biacetyl is reported. Following a dye laser pulse which prepares some of the biacetyl molecules in the triplet metastable state, the system is irradiated by an intense 20 ns 9.6μ CO2 pulse. The CO2 radiation induces fast quenching of the phosphorescence emission from the 3Au excited molecules. It also induces an emission signal in the fluorescence spectral region of biacetyl. This signal is related to an inverse electronic relaxation (IER) from excited triplet vibronic levels into isoenergetic singlet 1Au vibronic levels. Analysis of the induced luminescence signals provides information on the collisionless MP prompted vibrational distribution. Excitation with 10.6μ CO2 pulses leads to the simultaneous MP pumping of both the ground and triplet manifolds. The generation of blue emission signals in this experiment bears a close resemblance to recent observations of prompt visible emission due to MP pumping of ground state molecules. General expressions for the emission intensities are derived with special emphasis on the specific features of MP vibrational distributions. The detectability of MP induced emission signals is discussed.

Infrared multiphoton excitation and inverse electronic relaxation in SO2

Visible luminescence (λ=270–470 nm) has been observed from S16O2 and S18O2 at pressures of 0.2 to 20 Torr following irradiation by an intense infrared laser (λ=9.3 μm). Our experiments show that the luminescence is not due to dielectric breakdown or recombination of dissociation fragments, but rather is fluorescence from the first excited singlet states of SO2 following inverse electronic relaxation from highly excited vibrational levels of the ground electronic state. Crossover from the ground to excited electronic states may also be collisionally assisted. Spectroscopic and kinetic measurements are consistent with previous studies on 1B1 emission from SO2. The pressure dependence of the fluorescence yield exhibits two distinct pressure regimes, while the dependence of visible emission on laser pump wavelength follows the small signal infrared absorption spectrum. The threshold for detection of fluorescence is 17–20 J/cm2 with 9.3 μm radiation as the excitation source. These observations are discussed in terms of recently proposed theories which describe the photophysics of vibrationally excited states coupled to a radiative continuum through higher electronic states.

Multiphoton Induced Inverse Electronic Relaxation

High order infrared multiphoton excitation in the ground electronic state of collision free CrO2Cl2 has resulted in visible fluorescence. The prompt fluorescence has been studied as a function of laser fluence, pulse duration, under collisional and collision-free conditions and was shown to arise from a spontaneous one-photon radiative decay of molecular eigenstates. Intramolecular scrambling of vibronic levels corresponding to the ground state electronic manifold with a discrete level(s) belonging to the low lying excited electronic state is believed to be the origin of such eigenstates. This fluorescent channel is shown to compete with a dissociative channel forming CrO2Cl + CI; the radical further dissociates to the stable product Cr02. The threshold for fluorescence versus dissociation is discussed and a branching ratio, and consequent chemical mechanism are suggested.

Theory of inverse electronic relaxation

In this paper we advance a theory of inverse electronic relaxation (IER) induced by high-order multiphoton excitation of collision-free molecules. The IER process involves spontaneous one-photon radiative decay of molecular eigenstates. These states originate from intramolecular scrambling of vibronic levels corresponding to the ground state electronic manifold with a discrete vibronic level (or a set of such levels) which belong to a low lying excited electronic state. For a diatomic molecule and for a small polyatomic the molecular eigenstates are excited by a coherent multiphoton excitation process, while for larger polyatomic molecules where the low electronically excited state corresponds to an intermediate level structure or to the statistical limit, incoherent multiphoton excitation of the molecular eigenstates of an ’’isolated’’ molecule prevails. Explicit expressions for the rate of IER are derived and the conditions for the observation of this novel phenomenon are established.

Inverse electronic relaxation

We explore the possibility of detecting fluorescence emission following high-order infrared multiphoton excitation in the ground electronic manifold of polyatomic molecules and formulate the conditions for the experimental observation of such inverse electronic relaxation process.