Direct observation of rotational energy transfer in ammonia by time-resolved infrared—microwave double resonance

An N₂O laser is used to pump the ground vibrational state (8,7) inversion doublet of ¹⁴NH₃ while simultaneously monitoring other ground state doublets. Time-resolved rotational energy transfer signals are observed in accordance with known selection values. Absolute rates of rotational energy transfer processes are estimated.     

State‐state transitions for CCl2(X1A1, A1B1, a3B1) radical and collisional quenching of CCl2(A1B1 and a3B1) by O2, N2, NO,N2O,NH3, and various aminated molecules

CCl₂ free radicals were produced by a pulsed dc discharge of CCl₄ in Ar. Ground electronic state CCl₂(X) radicals were electronically excited to the A¹B₁ (0,4,0) vibronic state with an Nd:YAG laser pumped dye laser at 541.52 nm. Experimental quenching data of excited CCl₂(A¹B₁ and a³B₁) by O₂, N₂, NO, N₂O, NH₃, NH(CH₃)₂, NH(C₂H₅)₂, and N(C₂H₅)₃ molecules were obtained by observing the time‐resolved total fluorescence signal of the excited CCl₂ radical in a cell, which showed a superposition of two exponential decay components under the presence of quencher. The quenching rate constants k_A of CCl₂(A) state and k_a of CCl₂(a) state were derived by analyzing the experimental data according to a proposed three‐level model to deal with the CCl₂(X¹A₁, A¹B₁, a³B₁) system. The formation cross sections of complexes of electronically excited CCl₂ radicals with O₂, N₂, NO, N₂O, NH₃, and aminated molecules were calculated by means of a collision‐complex model. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 351–356, 2002     

Excited state dynamics of C2O(A3Πi)

The fluorescence decay and bimolecular electronic quenching behavior of C₂O ($\text{A}$³Π_i) is reported. C₂O($\text{X}$³Σ^?) is produced by laser photolysis of C₃O₂ at 266 nm and is subsequently excited by a tunable flashlamp pumped dye laser. The fluorescence decay is highly nonexponential and dominated by both short (≈ 15 μs) and long (50–250 μs) decay components. The long-lived emission, itself, is nonexponential. The fluorescence decay is modeled as the sum of three exponential components. The short-lived emission is quenched by C₃O₂ at higher than the gas kinetic rate while the long-lived fluorescence is quenched much less efficiently. Fluorescence quenching measurements are also reported for collisions with Ar, N₂ and O₂.     

Photoisomerization Mechanism of Ruthenium Sulfoxide Complexes: Role of the Metal‐Centered Excited State in the Bond Rupture and Bond Construction Processes

Phototriggered intramolecular isomerization in a series of ruthenium sulfoxide complexes, [Ru(L)(tpy)(DMSO)]ⁿ⁺ (where tpy=2,2’:6’,2’’‐terpyridine; DMSO=dimethyl sulfoxide; L=2,2’‐bipyridine (bpy), n=2; N,N,N’,N’‐tetramethylethylenediamine (tmen) n=2; picolinate (pic), n=1; acetylacetonate (acac), n=1; oxalate (ox), n=0; malonate (mal), n=0), was investigated theoretically. It is observed that the metal‐centered ligand field (³MC) state plays an important role in the excited state S→O isomerization of the coordinated DMSO ligand. If the population of ³MC_S state is thermally accessible and no ³MC_O can be populated from this state, photoisomerization will be turned off because the ³MC_S excited state is expected to lead to fast radiationless decay back to the original ¹GS_S ground state or photodecomposition along the Ru²⁺?S stretching coordinate. On the contrary, if the population of ³MC_S (or ³MC_O) state is inaccessible, photoinduced S→O isomerization can proceed adiabatically on the potential energy surface of the metal‐to‐ligand charge transfer excited states (³MLCT_S→³MLCT_O). It is hoped that these results can provide valuable information for the excited state isomerization in photochromic d⁶ transition‐metal complexes, which is both experimentally and intellectually challenging as a field of study.     

Vibrational relaxation induced population inversions in laser pumped polyatomic molecules

Conditions for population inversion in laser pumped polyatomic molecules are described. For systems which exhibit metastable vibrational population distributions [slow vibration—translation/rotation (V—T/R) relaxation], large, long lived inversions are possible even when the vibrational modes are strongly coupled by rapid collisional vibration—vibration (V—V) energy transfer. Overtone states of a hot mode are found to invert with respect to fundamental levels of a cold mode even at V—V steady state. Inversion persists for a V—T/R relaxation time. A gain of 4 m^?1 for the 2v₃ → v₂ transition in CH₃F (λ ≈ 15.9 μ) was found assuming a spontaneous emission lifetime of 10 s for this transition. General equations are derived which can be used to determine the magnitude of population inversion in any laser pumped, vibrationally metastable, polyatomic molecule. A discussion of factors controlling the population maxima of different vibrational states in optically pumped, V—V equilibrated metastable polyatomics is also given.     

Spectroscopic detection of protonated N2O

The high-resolution infrared spectrum ofprotonated N₂O has been observed with a difference frequency laser system. Protonated N₂O is generated in a hollow cathode discharge through a mixture of N₂O (≈ 15 mTorr) and H₂, (≈ 250 mTorr). It is found to be a bent molecule similar to HN₃, but it is not clear from this spectroscopic observation which isomer, O-protonated or N-protonated, has been detected. The molecular constants have been obtained for the ground state. The band origin of the V₁ fundamental is 3330.91070 cm⁻¹. Only the effective molecular constants have been given for the excited state due to a perturbation.     

Microwave spectral emission from a glow discharge-filled 18–26 GHz fabry-perot cavity spectrometer

A Fabry-Perot semiconfocal cavity resonator containing air at reduced pressure forms part of a Stark-modulated microwave spectrometer operated near the water 22.235-GHz (6₁₆–5₂₃) and ammonia 23.870 GHz (J = 3, K = 3) absorption lines. A glow discharge is formed in the cavity by an 8.333-kHz sinusoidal electric field of strength up to 140 kV m^?1. Under these conditions in the pressure region near 0.01–1 mbar, microwave emission from water and ammonia is observed when microwave energy between 4 to 6 dBm (source output power) at frequencies near that of the line is coupled into the cavity. Spectral features indicate that the emitting molecules arise from recombination of H + OH and H + NH₂. Predischarge features indicate that H₂O is excited directly by electron impact with simultaneous dissociation to OH and its excitation. NH₃ appears to be excited indirectly by energy transfer possibly from a metastable state resulting in dissociation to radicals in the discharge. Significant signal/noise treatment of the signals is found, compared with signals arising from coventional rotational (H₂O) or inversion (NH₃) absorption at these lines.     

Laser photolysis of ozone in the presence of ammonia. Formation and decay of vibrationally excited NH2 radicals

NH₂ radicals were generated by laser pulsed photolysis of O₃ in the presence of NH₃ and their subsequent kinetics were monitored by LIF. The room-temperature rate constant for O(¹D)+NH₃ was measured to be (3.3±0.1)×10^−1u cm⁻³ molecule⁻¹ s⁻¹ by studying NH₂ accumulation profiles. The nascent energy distribution over NH₂ vibrational levels was studied and the fraction of NH₂ radicals generated in the ground vibrational state was found to be 0.42±0.08. Acceleration of the reaction NH₂+O₃ caused by vibrational excitation of NH₂ radicals was observed - the rate constants were (1.5±0.4)×10⁻¹² and (1.5±0.3)×10⁻¹³ cm⁻³ molecule⁻¹ s⁻¹, for vibrationally excited and non-excited NH₂, respectively. The reactive channel for removal of vibrationally excited NH₂ by ozone appears to be dominant.     

Rotationally resolved laser-induced fluorescence and zeeman quantum beat spectroscopy of the V 1B2 state of jet-cooled CS2

The rotationally resolved laser-induced fluorescence (LIF) excitation spectrum of V system bands (V¹B₂≈X¹Σ¹_g transition) of CS₂ cooled in a supersonic jet has been observed. In a supersonic jet of CS₂/Ar or He mixture, the rotational temperature of CS₂ is reduced to less than 10 K, and thus the LIF excitation spectrum is simplified significantly. Two types of rotational structure are found; one is composed of P and R branch transitions from even J″ levels and the other is of P, Q, and R branch transitions from even as well as J″. The bands with the former rotational structure are assigned to transitions to K′ = O levels of ¹B₂ state, the bands with the latter structure to transitions to K′ = 1 levels from the (O, 1¹, O) level of the electronic ground state, i.e. vibrationally hot bands. This assignment is supported by the further evidence that these hot bands disappear when the supersonic jet includes a third-body gas such as NH₃ which enhances the vibrational relaxation of CS₂. Calculation of transition moments for respective leads to the conclusion that the upper levels of the V system bands are located in the region close to or higher than the potential barrier of the bending vibration of excited CS₂. The radiative lifetime of CS₂ in single rovibronic levels of the ¹B₂ state is in the range of 2–8 μs which is of the same order of magnitude as that calculated from the absorption coefficient. It tends to be longer for higher J levels or for higher vibronic levels. Zeeman quantum beating is observed in the fluorescence decay of excited CS₂ for a number of rovibronic levels under a weak magnetic field, and thus a magnetic moment associated with each rovibronic level can be determined. The g values are around 0.02 and tend to be smaller in higher J levels for some vibronic states. Based on the the observed radiative lifetime and the g value, it is suggested that the ¹B₂ state is perturbed by a spin-rotation interaction with two spin components, A₁ and B₁ of the ³A₂ orbital state besides a strong spin-orbit coupling with the R ³B₂ state.     

Excimer formation mechanism in gaseous krypton and Kr/N2 mixtures

A new approach for the understanding of the energy relaxation dynamics of excited atoms involving a long-lived molecular precursor is presented here for krypton. Excitation of the gas close to the 5s[3/2]₂ metastable atomic level (E _at. ?E _exc.<kT) is achieved with an intense VUV laser source (I ≈ 10¹² photon/pulse) realized by resonantly enhanced 4-wave mixing (2ω₁ + ω₂) in room temperature mercury vapor (N _Hg ≈ 10¹³ at./cm³). The decay of the II. continuum luminescence (145 nm) is studied. In the pressure range 200–500 mbar, decay rates depend linearly on pressure but have a negative zero-pressure intersect. We show here that this result can be understood as an effect of the exchange of energy between two different “reservoirs” of atomic (5s[3/2]₂) and molecular (1_g) nature, and can be an inherent peculiarity of the recombination kinetics of excited atoms with several product channels. The efficiency of the model is checked for the Kr/N₂ system. Rate constants for relaxation processes are determined in pure krypton and in Kr/N₂ mixtures.     

The energy balance and branching ratios associated with the chemiluminescent reaction Si(3P) + N2O(1Σ) → SiO*(a3Σ+, b3Π, A1Π) + N2(υ″ ⩾ 5) — possible formation of vibrationally excited N2

Silicon atoms react under single collision conditions with N₂O to yield chemiluminescent emission corresponding to the SiO a³Σ⁺?X¹Σ⁺ and b³Π?X¹Σ⁺ intercombination systems and the A¹Π?X¹Σ⁺ band system. A most striking feature of the SiN₂O reaction is the energy balance associated with the formation of SiO product molecules in the A¹Π and b³Π states. A significant energy discrepancy ( = 10000 cm^? = 1.24 eV) is found between the available energy to populate the highest energetically accessible excited-state quantum levels and the highest quantum level from which emission is observed. It is suggested that this discrepancy may result from the formation of vibrationally excited N₂ in a concerted fast SiN₂O reactive encounter. Emission from the SiO a³Σ⁺ (A¹Π) and b³Π(A¹Π, E¹Σ₀⁺) triplet-state manifold results primarily from intensity borrowing involving the indicated singlet states. Perturbation calculations indicate the magnitude of the mixing between the b³Π, A¹Π and E¹Σ₀⁺ states ranges between 0.5 and 2%. On the basis of these calculations, the branching ratio (excited triplet)/(excited singlet) is found to be well in excess of 500. An approximate vibrational population distribution is deduced for those molecules formed in the b³Π state. The present studies are correlated with those of previous workers in order to provide an explanation for diverse relaxation effects as well as observed changes in the ratio of a³Σ⁺ to b³Π emission as a function of pressure and experimental environment. Some of these effects are attributable to a strong coupling between the a³Σ⁺ and b³Π state. Based on the current results, there appears to be little correlation between either (1) the branching ratio for excited state formation or (2) the total absolute cross section for excited-state formation and (3) the measured quantum yield for the SiN₂O reaction. Implications for chemical laser development are considered.     

Extremely fast radiationless transition from the triplet state of 9,10-diazaphenanthrene

The decay of the lowest excited triplet state of 9,10-diazaphenanthrene (DAP) was investigated. A surprisingly high sublevel decay rate (k_y = 3.3 × 10⁷ s⁻¹) in biphenyl and fluorene hosts was found at 3.0 K from the decay of a time-resolved EPR (TREPR) signal. This high decay rate was confirmed by a lifetime measurement of the delayed fluorescence using a picosecond laser and photon counting system. The nature of the fast radiationless transition is discussed.     

“Magic” losses of one to five N2 molecules from metastable N+2n clusters

SIMS and FAB of solid N₂ generated large N_2n⁺ clusters (N₄⁺ -N⁺₅₄) which showed three types of metastable decay: (i) loss of a single N₂ molecule, assigned to ordinary (“thermal”) metastability, (ii) nearly total disintegration, assigned to radiationless decay of an electronically excited state, and (iii) “magic” losses of very specific small numbers of N₂ molecules, assigned to N₂(v = 1) vibrational relaxation by use of isotope effects. Average N₂ binding energies are estimated.     

On collision-induced amplified emission of alkali atoms

The recently discovered amplified emission of alkali atoms resonantly excited to theP _3/2 state by intense laser beam is analyzed theoretically. This effect is caused essentially by collisions with buffer gas atoms building up a population inversion of theP _1/2 state with respect to theS _1/2 ground state. Our theoretical calculations based on dressed-atom density matrix analysis agree with most experimental results.     

Photon-echo decay of naphthalene in durene and perdeutero-naphthalene

Using two frequency-doubled, nitrogen laser pumped, dye-lasers we have measured the decay of the photon-echo in the lowest ¹B_1u ← ¹A_1g singlet state of naphthalene in durene and in perdeutero-naphthalene. Optical phase relaxation in the durene mixed crystal is believed to be caused by resonant phonon scattering in the ground and excited state, and in the isotopically mixed crystal by scattering of the excited state into the singlet exciton band. At the lowest achievable temperature (1.4 K), the photon-echo decay time (T₂) in both systems is still found to be much shorter than expected from the fluorescence decay times. Energy transfer is held responsible for this discrepancy.     

Luminescence spectrum and kinetics of laser-excited HgNH3 complex

When an Hg/NH₃ gas mixture was irradiated by laser light around 260 nm, luminescence appeared in the range of 290–360 nm. The spectrum of the luminescence was almost the same as that induced by the Hg photosensitized reaction of NH₃, and thus, the laser-induced luminescence was assigned to emission of an excited HgNH₃ complex. The luminescence spectrum as well as the excitation spectrum could be reproduced by the Franck-Condon factors calculated from the bound-free transition between the Hg-NH₃ repulsive ground state and the excited bound state. Time-evolution measurements on the luminescence induced by the pulsed irradiation of the laser indicated that initially formed HgNH₃ excimer had to make a collisional transition to a state which could emit the luminescence around 340 nm.     

Picosecond diffuse reflectance and transmission laser flash photolysis study of various triaryl-2-pyrazolines

In this study the first ever reported application of diffuse reflectance laser flash photolysis for the observation of sub-nanosecond transient absorption decays is presented. The compounds studied are various triaryl-2-pyrazolines, both as microcrytals and contained within polycarbonate films. The microcrystalline samples were studied using pump—probe laser flash photolysis in diffuse reflectance mode and the observed transient absorption decay could be fitted using a biexponential model with, in the case of 1, 5-diphenyl-3-styryl-2-pyrazoline, lifetimes of 1.6 × 10⁻¹⁰ and 1.3 × 10⁻⁹s for the first and second decay components, respectively. This model could also be used to fit the decay kinetics obtained from transmission pump—probe laser flash photolysis experiments conducted upon polycarbonate films containing this same compound, the lifetimes in this instance being 5.5 × 10⁻¹² and 1.7 × 10⁻¹⁰s for the first and second decay components, respectively. In addition, a study of the quenching of the pyrazoline excited states in a polycarbonate matrix by disulphone magenta was undertaken. In this case it was necessary to modify the second term of the biexponential model with a term to allow for Förster type long range energy transfer, the Förster critical transfer distance being determined as 25 Å. This biexponential model is rationalized as initial excitation being to the S₂ state, the first decay component being relaxation to the S₁ state and the second component decay of the S₁ state to the ground state, by radiative and non-radiative relaxation and, when DSM is present, long range energy transfer to this energy acceptor.     

Fluorescence decay kinetics of acetone vapour at low pressures

Acetone-h₆ and -d₆ were excited by a short UV laser pulse to the nπ* state. Using pressures of 10^?4-10 ^?3 Torr, two distinct decay components were observed - the faster with a decay time of less than 20 ns and the slower of about 5 μs. Increasing the pressure leads to the appearance of two longer-lived decay components, which are apparently absent in the case of isolated molecules. Based on the deuteration effect, excitation wavelength dependence, quenching kinetics and analogy with other molecules, the four decay components are assigned as follows. The fastest component is due to dephasing of the initially excited state, forming a quasi-stationary eigenstate. The second component is due to the radiative decay of the latter states. The third, to decay of triplet states not directly coupled to the initially excited singlet states, and the last to the thermalized triplet state.     

Collisional de-excitation of the metastableD-states of Ba+ by He,Ne, N2 and H2

The rate constants 〈σ · υ〉 for collisional de-excitation of the metastable 5D states of Ba⁺ ions have been determined in an ion trap experiment. TheD-states are selectively populated by pulsed laser excitation of the 6P _1/2 or 6P _3/2 state and the decay at different background pressures is monitored by the change in fluorescence intensity of the excited ions. From the pressure dependence of the decay constants we calculate the de-excitation rate constants for different collision partners, averaged over the velocity distribution of the trapped ion cloud. For He, Ne, H₂ and N₂ we obtain in the c.m. energy range of 0.1–0.5 eV: 〈σ·υ〉 (He)=3.0±0.2·10^?13cm³/s, 〈σ·υ〉 (Ne)=5.1±0.4·10^?13cm³/s, 〈σ·υ〉 (H₂)=3.7±0.3·10^?11cm³/s, 〈σ·υ〉 (N₂)=4.4±0.3·10^?11cm³/s. The results can be understood qualitatively by a consideration of the ion-atom and ion-molecules interaction potential.     

Nanoseconds UV laser study of radiationless deactivation from upper electronic excited states in solution: Tb3+

Using 25 ns half width pulses of a frequency-quadrupled Nd laser at 265 nm Tb³⁺ perchlorate in D₂O solution, or in borate glass, was excited to high electronic states and the fluorescence on transition from the upper ⁵D₃ and lower ⁵D₄ excited states to various sub-levels of the lowest ⁷F state were measured. The appropriate energy gap spacing of this rare earth ion results in lifetimes which enabled us to observe the decay of ³D₃ as well as the growing in and decay of ⁵D₄. Radiationless processes from ⁵D₃ direct to ⁷F or cascading to ⁵D₄ are discussed and correlated with previous theoretical and experimental results on related systems involving the interaction of the non-complexed solvated ion with the medium, and compared with related phenomena in glassy solids.