Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: A (2)Sigma(v(') = 0)<--X (2)Pi(v(") = 0,1,2,3). Simulations of the NO rovibronic intensities for the A<--X transitions show that dissociated NO from RDX and HMX is rotationally cold (approximately 20 K) and vibrationally hot (approximately 1800 K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.     

Intermolecular vibrations and dynamics of the anisole—benzene complex studied by multi-resonance spectroscopy

The intermolecular vibrations of the anisole—benzene complex in the ground and excited electronic states have been observed by the LIF (laser-induced fluorescence) and fluorescence-dip techniques. Short progressions due to the intermolecular vibrations suggest a small structure change of the complex upon electronic excitation. The LIF excitation spectrum shows predominant progressions of 27 cm⁻¹, which is tentatively assigned to one of the intermolecular bending modes in the excited electronic state. On the other hand, the fluorescence-dip spectrum shows only a series of bands with irregular intervals due to the intermolecular modes in the ground electronic state. The decay rates of the vibrationally excited complex in the ground electronic state have also been measured with the SEP-LIF (stimulated emission pumping-laser-induced fluorescence) technique, where the complex vibrationally excited by SEP is probed by the delayed LIF measurements. The complex excited to its purely intermolecular mode stays in the initially prepared state after a delay time of 1 μs. On the other hand, the complex excited to the intramolecular vibrational states above 500 cm⁻¹ does not seem to stay in the prepared states. Neither the relaxed complex nor the dissociated monomer was detected. A possible reason for this observation is discussed.     

Quantum state-resolved collision relaxation of highly vibrationally excited SO2

Collision depopulation cross sections of 13 single, highly vibrationally excited levels with 45,000 cm(-1) energy in the electronic ground state of SO(2) in collision with CO in a supersonic jet have been measured. The measurements for these single highly excited quantum states are conducted through pressure dependence of the decay of the fluorescence quantum beat resulted from their coupling with the rovibronic levels in the optically allowed transitions to the (140), (210), and (132) C(1)B(2) levels. The relaxation cross sections of these highly excited states, each with well-defined energy and symmetry, range from 27 to 187 A(2) with an average of 71 A(2). This average cross section is much larger than the hard sphere cross section of 48 A(2). The relaxation cross section is also found to be larger for the quantum states with a larger matrix element in coupling with the "bright" electronically excited level. Both observations suggest a substantial contribution from long range interactions in collision relaxation of highly excited molecules.     

Intense-field modulation of NO2 multiphoton dissociation dynamics

We report on the dynamics of multiphoton excitation and dissociation of NO(2) at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm(-2). The breakup of the molecule is monitored by NO A (2)Sigma(+)n(')=1,0-->X (2)Pi(r)n(")=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n(')=0 and 1 NO A (2)Sigma(+) results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO(2) valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A (2)Sigma(+)n(')=0-->X (2)Pi(r)n(")=0 fluorescence are consistent with delayed generation of NO A (2)Sigma(+)n(')=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed A (2)B(2) and Rydberg 3ssigma (2)Sigma(g) (+) states of the parent molecule. The increasingly prompt decay of A (2)Sigma(+)n(')=1-->X (2)Pi(r)n(")=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X (2)A(1) and 3ssigma (2)Sigma(g) (+) dressed states to generate vibrationally excited products.     

Intramolecular photoexcitation dynamics and magnetic field effects in an intermediate-case molecule

Molecular vibration and rotation play a significant role in the intramolecular photoexcitation dynamics of the so-called intermediate-case molecule, and the fluorescence intensity, decay and polarization of s-triazine vapor are shown to depend on the excited rovibronic level of the S₁ state. Fluorescence characteristics are interpreted by assuming three zero-order states: (1) a zero-order singlet state that carries the absorption intensity and emits fluorescence with sharp structure; (2) zero-order singlet states that do not carry the absorption intensity but emit broad fluorescence; and (3) zero-order triplet states. The interaction among these states depends not only on the vibrational level but also on the rotational level excited. It is suggested that the number of triplet states coupled to the singlet state increases with increasing excess vibrational energy. It is also suggested that K-scrambling occurs both in the triplet manifold following intersystem crossing (ISC) and in the singlet manifold following intramolecular vibrational energy redistribution (IVR). The fluorescence intensity and decay of s-triazine vapor are significantly influenced by a magnetic field, and the field effects are interpreted in terms of the spin decoupling in the triplet manifold following ISC; the role of external magnetic fields is to mix the spin sublevels of different rovibronic levels coupled to the excited singlet state. Magnetic depolarization of fluorescence also occurs because of the efficient interaction between the excited singlet state and the triplet state.     

A dispersed fluorescence and ab initio investigation of the X2B1 and A2A1 electronic states of the PH2 molecule

In this work, the X2B1 and A2A1 electronic states of the phosphino (PH2) free radical have been studied by dispersed fluorescence and ab initio methods. PH2 molecules were produced in a molecular free-jet apparatus by laser vaporizing a silicon rod in the presence of phosphine (PH3) gas diluted in helium. The laser-induced fluorescence, from the excited A2A1 electronic state down to the ground electronic state, was dispersed and analyzed. Ten (upsilon1upsilon2upsilon3) vibrationally excited levels of the ground electronic state, with upsilon1 < or = 2, upsilon2 < or = 6, and upsilon3 = 0, have been observed. Ab initio potential-energy surfaces for the X2B1 and A2A1 electronic states have been calculated at 210 points. These two states correlate with a 2Pi(u) state at linearity and they interact by the Renner-Teller coupling and spin-orbit coupling. Using the ab initio potential-energy surfaces with our RENNER computer program system, the vibronic structure and relative intensities of the A2A1 --> X2B1 emission band system have been calculated in order to corroborate the experimental assignments.     

Electronic spectrum of the AlC(2) radical

An electronic transition of the AlC2 radical (C2v structure) has been observed using laser-induced fluorescence spectroscopy. The molecule was prepared in a supersonic expansion by ablation of an aluminum rod in the presence of acetylene gas. A spectrum was recorded in the 451-453 nm region and assigned to the C 2B2-X 2A1 system (T0 = 22,102.7 cm(-1)) based on a rotational analysis and agreement with calculated molecular parameters and excitation energies. Ab initio results obtained using couple cluster methods are in accord with previous theoretical work which concludes that ground-state AlC2 possesses a T-shaped C2v 2A1 geometry, with the linear 2Sigma+ AlCC isomer 0.70 eV higher in energy. A fit of the experimental spectrum yields rotational constants in the ground and electronically excited states that are in reasonable agreement with the calculated values: A' = 1.7093(107), B' = 0.4052(50), C' = 0.3228(49) cm(-1) for the X 2A1 state, and A' = 1.5621(137), B' = 0.4028(46), C' = 0.3201(54) cm(-1) for C 2B2. Variation in individual fluorescence lifetimes suggests that the emitting C 2B2 state undergoes rovibronic mixing with lower lying electronic states.     

Ultrafast fluorescence detection in tris(2,2'-bipyridine)ruthenium(II) complex in solution: relaxation dynamics involving higher excited states

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.     

Laser spectroscopy and dynamics of the jet-cooled AsH2 free radical

The A (2)A(1)-X (2)B(1) electronic transition of the jet-cooled AsH(2) free radical has been studied by laser-induced fluorescence (LIF), wavelength-resolved emission, and fluorescence lifetime measurements. The radical was produced by a pulsed electric discharge through a mixture of arsine (AsH(3)) and high pressure argon at the exit of a pulsed valve. Nine vibronic bands were identified by LIF spectroscopy in the 505-400 nm region, including a long progression in the bending mode and two bands (1(0) (1) and 1(0) (1)2(0) (1)) involving the excited state As-H symmetric stretch. Single vibronic level emission spectra showed similar activity in the bending and symmetric stretching frequencies of the ground state. High-resolution spectra of the 0(0) (0) band exhibited large spin splittings and small, resolved arsenic hyperfine splittings, due to a substantial Fermi contact interaction in the excited state. The rotational constants obtained in the analysis gave effective molecular structures of r"(0)=1.5183(1) A, theta"(0)=90.75(1) degrees and r'(0)=1.4830(1) A, theta'(0)=123.10(2) degrees . The excited state fluorescence lifetimes vary dramatically with rovibronic state, from a single value of 1.4 micros to many with lifetimes less than 10 ns, behavior which the authors interpret as signaling the onset of a predissociative process near the zero-point level of the ground state.     

紫外光激励产生高振动激发ν_1模的NO_2分子

NO2的光解研究不仅在大气化学等领域有着重要意义，而且在单分子解离模型的建立等分子反应动力学理论研究上也有着重要的价值．因此，它一直被不同的实验方法进行着深入的研究．NO2的解离极限波长是398nm．由干室温下NO2振、转动能的能量贡献，在长干解高限的404．7。仍有多达3     

紫外光激励产生高振动激发ν1模的NO2分子

The photon-excited NO2 at 308 nm has been investigated by Time-Resolved FTIR spectroscopy. The IR fluorescence from highly excited NO2(X2 A1) in ν1 vibrational mode has been observed. These excited states are resulted from the strong vibronic mixing of electronic excited A2 B2/B2 B1 states with the ground X2 A1 state. It is considered that symmetric stretching ν1 mode is reserved from the photolysis because its vibrational style is unsuitable for dissociation.     

Electronic energy levels in all-trans long linear polyenes: the case of the 3,20-di(tert-butyl)-2,2,21,21-tetramethyl-all-trans-3,5,7,9,11,13,15,17,19-docosanonaen (ttbp9) conforming to Kasha's rule

The absorption, fluorescence and fluorescence excitation spectra for 3,20-di(tert-butyl)-2,2,21,21-tetramethyl-all-trans-3,5,7,9,11,13,15,17,19-docosanonaen (ttbP9) in dilute solutions of 2-methylbutane were recorded at temperatures over the range 120-280 K. The high photostability of this nonaene allows us to assert that it exhibits a single fluorescence and that this can be unequivocally assigned to emission from its 1(1)B(u) excited state, it being the first excited electronic state. Available photophysical data for this polyene and the wealth of information reported for shorter all-trans polyenes allow us to conclude that if the first excited electronic state for the chromophore possessed 2(1)A(g) symmetry, then the energy of such a state might have been so close to that of the 1(1)B(u) state that: 1) the radiationless internal conversion mechanism would preclude the observation of the emission from the 1(1)B(u) state reported in this work and 2) the 2(1)A(g) state reached through internal conversion would be vibrationally coupled to 1(1)B(u) and would facilitate the detection of the emission from 2(1)A(g), which was not observed in any of the solvents used in this work. The spectroscopic and photochemical implications of these findings for other polyenes are discussed.     

Lifetime measurements of SO2 below the Clement's A-band

High resolution laser induced fluorescence spectrum of jet-cooled SO(2) is recorded toward the blue side of the Clement's A-Band in the region of 314-319 nm. Time resolved fluorescence measurements have been carried out for all the prominent peaks in this region. Most of the peaks exhibited double exponential decay profiles. Some of the rovibronic bands exhibited quantum beats with strong quantum beats observed at 315.261 and 315.271 nm. This is the first observation of quantum beats in SO(2) in the absence of any external magnetic or electric fields. The decay profiles of the beating rovibronic bands were fitted using a four-level model by least-squares fitting method. The fitting shows that all the measured bands were double exponential with a similar first lifetime of approximately 3 mus and a varying second lifetime of the order of 1 micros-100 ns with a beating frequency of approximately 1 MHz. These quantum beats, in the absence of any external field, indicate rotational level mixing between the A (1)A(2) and the B (1)B(1) vibronic states which are near resonant due to the high density of states of these two states.     

Structure and Decay of Gaseous Organic Radical Cations Studied by Their Radiative Decay,Exemplified by the 1,3-Pentadiyne Cation

The radiative decay of over a hundred open-shell organic radical cations has now been established. As a result, the spectral structure of such cations in their ground and excited electronic states can be probed with resolutions of the order of ? 1 cm^?1. This is achieved by means of emission and laser-induced fluorescence techniques. The analysis of the emission and excitation spectra provides the vibrational frequencies of many of the totally symmetric fundamentals of the cations in the two electronic states. In order to study the relaxation behavior of these cations under “isolated conditions”, the lifetimes and fluorescence quantum fields can be obtained by means of photoelectron-photon coincidence measurements. These data yield the radiative and non-radiative rate constants as a function of the internal energy of the cations. The structural and decay information obtained from each of these techniques is illustrated using the 1,3-pentadiyne radical cation as example.     

Experimental Study of C2Cl3+NO2 Reaction

The free radical reaction of C2Cl3 with NO2 was investigated by step-scan time-resolved FTIR (TR-FTIR) emission spectroscopy. Due to the vibrationally excited products of Cl2CO, NO, and CO, strong IR emission bands were observed with high resolution TR-FTIR spectra. Four reaction channels forming C2Cl3O+NO, CCl3CO+NO, CO+NO+CCl3, and ClCNO+Cl2CO were elucidated, respectively. Spectralˉtting showed that the product CO was highly vibrationally excited with the nascent average vibrational energy of 60.2 kJ/mol. Possible reaction mechanism via intermediates C2Cl3NO2 and C2Cl3ONO was proposed.     

Emission characteristics of the dielectronic recombination line of atomic Cu and selectively excited ionic Cu line by resonance charge-transfer collision in a low-pressure laser-induced plasma.

Emisson spectra and time-resolved two-dimensional (2D) emission images of the electron-ion dielectronic recombination (i.e. a reversal process of auto-ionization) line of neutral Cu atoms, the selectively excited Cu ionic line, and normal Cu atomic line were observed for understanding the excitation mechanisms of Cu neutral and ionic lines in a low-pressure laser-induced plasma (LP-LIP) of Ar. From the observations, the number of charged particles around the emitting species seems to increase with increasing Ar pressure. Different time-resolved 2D emission images were observed among the selectively excited Cu ionic line and other Cu emission lines resulting from the different excitation mechanisms of the respective emission lines. Collisions of the second kind and electron-ion recombinations were found to be one of the major excitation mechanisms of Cu in Ar LP-LIP.     

Rovibrational resonance effects in collision-induced electronic energy transfer: I2(E,v=0-2)+CF4

Collisions of I2 in the E(0(g)+) electronic state with CF4 molecules induce electronic energy transfer to the nearby D, beta, and D' ion-pair states. Simulations of dispersed fluorescence spectra reveal collision-induced electronic energy transfer rate constants and final vibrational state distributions within each final electronic state. In comparison with earlier reports on I2(upsilon(E)=0-2) collisions with He or Ar atoms, we find markedly different dynamics when I2, excited to the same rovibronic states, collides with CF4. Final vibrational state distributions agree with the associated Franck-Condon factors with the initially prepared state to a greater degree than those found with He or Ar collision partners and suggest that internal degrees of freedom in the CF4 molecule represent a substantial means for accepting the accompanying loss of I2 vibronic energy. Comparison of the E-->D transfer of I2 excited to the J=23 and J=55 levels of the upsilon(E)=0 state reveals the onset of specific, nonstatistical dynamics as the available energy is increased above the threshold for excitation of the low frequency nu2 bending mode of CF4.     

Photodissociation spectroscopy and dynamics of the CH2CFO radical

The photodissociation spectroscopy and dynamics resulting from excitation of the B (2)A(")<--X (2)A(") transition of CH(2)CFO have been examined using fast beam photofragment translational spectroscopy. The photofragment yield spectrum reveals vibrationally resolved structure between 29 870 and 38 800 cm(-1), extending approximately 6000 cm(-1) higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH(2)F+CO and HCCO+HF fragment channels are observed. Both product channels yield photofragment translational energy distributions that are characteristic of a decay mechanism with a barrier to dissociation. Using the barrier impulsive model, it is shown that fragmentation to CH(2)F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH(2)CFO and CH(2)FCO governing the observed translational energy distributions.     

Emission spectroscopy and excited-state dynamics of chromyl-chloride vapor

A tunable dye laser has been used to excite single vibronic features in the low-pressure vapor of CrO₂Cl₂. The fluorescence spectrum, fluorescence excitation spectrum and time-resolved fluorescence decay are discussed. It is shown that the active ν′₄ and ν″₄ modes are the same frequency in the gas phase, thus collapsing the sequence congestion normally observed in gas-phase spectra. This degeneracy makes impossible the excitation of single vibronic levels. It is shown that the fluorescence lifetime of the excited state in all except the vibrationally cold level is severely shortened by unimolecular radiationless decay. This radiationless rate is strongly dependent upon the partitioning of energy into various excited-state modes. The radiative lifetime of the vibrationally cold excited state is (1.34 ± 0.08) μs and the apparent bimolecular quenching rate is (5.9 ± 0.2) × 10^?10 cm³/molecules. No evidence of emission from the lowest-energy excited electronic state recently reported by Spoliti [J. Mol. Spectrosc. 52 (1973) 146] is observed.     

Spectroscopic properties of Pr3+ ions in a PbWO4 single crystal

Site-selective fluorescence laser spectroscopy of Pr (3+) ions in lead tungstate single crystal were investigated at temperatures from 10 to 300 K. The site-selective emission spectra and fluorescence decays from the (3)P J ( J = 0, 1, 2) and (1)D 2 states were analyzed. The (3)P J ( J = 0, 1, 2) level shows its predominantly radioactive character with the typical greenish-blue luminescence ascribed to (3)P J transition. The emission from the (1)D 2 level is only observed when this level is directly excited. The decay kinetic of the (1)D 2 level was measured under site-selective excitation and discussed in terms of cross-relaxation. The up-conversion emission from levels (3)P 1 and (3)P 0 following excitation of the (1)D 2 state was observed in the PbWO 4 crystal between 10 and 300 K. The main up-conversion mechanism, together with the understanding the quenching of the (1)D 2 fluorescence in this Pr (3+) heavily doped PbWO 4 were discussed. The presence of the complex structures of the emission spectra and different decay profiles indicate that several processes contribute to the quenching of the (1)D 2 fluorescence of Pr (3+) ions. It was found that the up-conversion fluorescence intensity had a quadratic dependence on the laser input power. The temporal behavior of the up converted emission indicates that an energy-transfer up-conversion is the dominant process.