Infrared multiphoton dissociation of vinyltrifluorosilane

Infrared multiphoton absorption and dissociation of vinyltrifluorosilane molecules under the action of pulsed TEA CO₂-laser were experimentally studied. The composition of the end dissociation products has been analyzed. Using quantum chemical calculations, we have identified the dissociation pathways that lead to the observed end products. Transition state geometries, enthalpies and activation energies for the dissociation pathways have been calculated. The dissociation channel ratio was determined under used experimental conditions. The silicon isotope selective infrared multiphoton dissociation has been performed at different wavelengths of the CO₂-laser radiation. PACS 82.50Bc; 82.30Lp; 31.15.Ew     

Infra-red multiphoton dissociation

Summary Threshold processes and ideally focused, non attenuated beams areconsidered. The fractional dissociation per pulse ω is plainly related, through the local dissociation probabilityP, to a local primitive functionS depending only on the radiation frequency ν, the maximum ε of the local fluence, the state variables β as well as, possibly, the pulse temporal structure; ω is found to scale with regard to pulse energy focal length and cell volume in typical ways provided certain defined conditions are satisfied. ε₀ being the empirical threshold, it may be realistic and convenient to simply assumeS≡0 for ε⩽ε₀ andS=s ₁(ε−ε₀) for ε>ε₀,i.e. a step cross-section , with for ε⩽ε₀ and for ε>ε₀. Species like³²SF₆ and¹²CF₂Cl₂ comply with this step crosssection approximation within large limits. In addition,³² s ₁ turns out to be independent of the pulse tenporal structure and to be factorable:³² s ₁(ν, β)=ν(ν)·β(β).

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Riassunto Si considerano processi a soglia in fasci non attenuati e perfettamente focalizzati. Mediante la probabilità locale di dissociazione, la frazione di molecole dissociate per imulso ω è messa in relazione con una funzione primitiva localeS che dipende solo dalla frequenza della radiazione ν, dal massimo della fluenza locale ε, dalle variabili di stato β ed, eventualmente, dalla struttura temporale dell’impulso; si trova che ω varia con l’energia dell’impulso, la distanza focale ed il volume della cella in modi caratteristici purché siano soddisfatte alcune condizioni. Se ε₀ è la soglia empirica, può risultare realistico e conveniente semplicemente ammettere cheS≡0 per ε⩽ε₀ e cheS=s ₁(ε−ε₀) per ε>ε₀, cioè che la sezione d’urto sia a soglia, con per ε⩽ε₀ ed per ε>ε₀. Questa approssimazione è adeguata entro ampi limiti per specie come³²SF₆ e¹²CF₂Cl₂. Inoltre si osserva che³² s ₁ è indipendente dalla struttura temporale dell’impulso e fattorizzabile:³² s ₁(ν,β)=ν(ν)·β(β).

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Резюме Расаматриваются пороговые процессы и идеально сфокусированные, незатухающие пучки. Молярная диссоциация за импульс ω связана, через вероятность локальной диссоциацииP, с локальной функциейS, зависящей только от частоты излучения ν, от максимума локального потока ε, от переменных состояния β, а также может быть от временной структуры импульса. Получено, что ω изменяется с знергией импуляса, фокусным расстоянием и объемом ячейки обычным образом, при выполнении определенных условий, ε₀ является змпирическим порогом. Можно предположить, чтоS≡0 для ε⩽ε₀ иS=s ₁(ε−ε₀) для ε>ε₀, т.е. поперечное сечение является ступенчатым , с при ε⩽ε₀ и при ε>ε₀. Образцы типа³²SF₆ и¹²CF₂Cl₂ в широких пределах удовлетворяют зтому ступенчатому поперечному сечению. Кроме того, оказывается, что³² s ₁ не зависит от временной структуры импульса и факторизуется:³² s ₁(ν,β)=ν(ν)·β(β).

     

Infrared multiphoton dissociation of13CF3Cl induced by CO2 laser pulses

Selective infrared multiphoton dissociation of¹³CF₃Cl induced by CO₂ laser pulses adjusted on = 1071.9 cm^–1 has been studied in the energy rangeE between 0.5 and 2 J per laser pulse or fluence range between 5 and 25 J per cm², and in the pressure range between 0.10 and 60 Torr. The effect of these parameters on the isotopic selectivity of the dissociation gives information on the rotational relaxation constants. As for the dissociation probabilities, they vary exponentially withE ^–1. The applicability of such an Arrhenius-type relation is discussed and semi-quantitatively justified.     

Quantum statistics of multiphoton unimolecular dissociation

Summary We suggest a statistical model for the process of multiphoton unimolecular dissociation which considers the energy levels of the vibrational active mode, a quasi-continuous heat bath constituted by the energy levels of the remaining modes of the molecule and an electronically excited fragment resulting from the dissociation. The master equation for the system and expressions for the moments are derived and discussed in the standard second-quantization formalism. From the master equation the diagonal and off-diagonal density matrix elements are evaluated and some discussion on their applications to establish the change of light statistics is given.

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Riassunto Si suggerisce un modello statistico per il processo di dissociazione unimolecolare multifotonico che considera i livelli di energia del modo attivo vibrazionale, un bagno di calore quasi continuo costituito dai livelli di energia dei restanti modi della molecola e di un frammento eccitato elettronicamente che risulta dalla dissociazione. L’equazione master del sistema e le espressioni degl’impulsi sono derivate e discusse nel formalismo standard della seconda quantizzazione. Per l’equazione master, gli elementi di matrice della densità diagonali e al di fuori della diagonale sono calcolati e si discute sulle loro applicazioni per stabilire il cambiamento della statistica della luce.

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Резюме Мя предлагаем статистическую модель для процесса многофотонной мономолекулярной диссоциации. Эта модель учитывает энергетические уровни колебательной активной моды, энергетические квазинепрерывные уровни термостата, которые состоят из знергетических уровней оставшихся мод молекулы, и электронных возбуюдений фрагментов, образованных при диссоциации. Выводится задаюЩее уравнение для системы и выражения для моментов. Проводится обсуждение в рамках стандартного формализма вторичного квантования. С помощью задающего уравнения вычисляются диагональные и недиагональные элементы матрицы плотности. Проводится обсуюдение статистики света.

     

Deuterium separation by multiphoton dissociation of dichlorofluoromethane

Absorption and dissociation probabilities of CHCl₂F and CDCl₂F were investigated by a pulsed CO₂ laser in the wavenumber region of largest selectivity for deuterium. The absorption of CHCl₂F, which is a difference band, can largely be suppressed by cooling. At 200 K and at 920 cm^–1 absorption selectivities up to 4000 were found by extrapolation. In the presence of buffer gas, CDCl₂F can be multiphoton excited nearly like a linear absorber (harmonic oscillator). This is interpreted by a nearly resonant collisional relaxation v₇ to v₂ and by the smallness of the cross anharmonicity x₂₇. The dissociation selectivityS was 24,000 at natural abundance. Such large values were measured by a chromatographic method.S depends only onp _D, the partial pressure of the deuterated species. This dependence is approximately ln Sp _D ^–1 . It can be rationalized by considering only the average energy transferred to the nonresonant molecules by collisions with CDCl₂F. The above functional shape is related to an Arrhenius type law. Comparison with trifluoromethane for D separation shows that CHCl₂F has primarily two advantages: its rapid H-D exchange with water and the less stringent requirements of laser energy and pulse length.A preliminary account of this work has appeared in Quantum Electr.2, 13 (1985) (in Chinese)     

Intensity dependence of multiphoton dissociation in formaldehyde

A new intensity-dependent measurement of multiple-photon dissociation (MPD) in H₂CO, HDCO, and D₂CO gases by the use of an intense pulsed CO₂TEA laser is reported. In this measurement the energy and duration of the laser pulses are kept constant, and the intensity is varied by irradiating the sample using concave mirrors of different focal lengths. A model calculation is used to analyze and fit the present and previous experimental MPD data of HDCO and D₂CO. In this model it is assumed that dissociation is obtained by a repeated mechanism in which coherent multiphoton excitation (CME) of the molecule to high vibration-rotation states |v, J〉 is followed by intramolecular transfer of the excitation energy (ITEE) to the other modes of the molecule. In the calculations the CME is described in the framework of the density matrix formalism with relaxations, and is used to calculate absorption from the ground state as well as absorption from excited states reached by the energy redistribution in the molecule. The ITEE process is assumed to be intensity independent and to cause a random energy distribution in each transferring process. It is found that the experimental results are consistent with the absorption of 14±4 and 17±5 photons per molecule for HDCO and D₂CO, respectively, and this is sufficient to cause their dissociation.     

Resonant multiphoton dissociation of the NO molecule

The multiphoton dissociation of an unexcited NO molecule involving intermediate Rydberg states n?σ(²∑⁺), n?π(²П), and n?δ(²Δ), which are populated by a weak probe field and decay owing to nonadiabatic transitions in the predissociation states of valence configurations B′²Δ, L ²П, I ²∑⁺, and B ²П, has been studied. The role of the ponderomotive interaction that occurs under the simultaneous action of an intensive electromagnetic monochromatic radiation on the studied system has been analyzed. A theory to describe the dynamics of the process has been described. The possibilities of laser stimulation of this process, which consist in the search for ranges of variation in the frequencies and intensities of laser radiation that provide the highest efficiency of the resonant photostimulated process, have been discussed.     

Infrared multiphoton excitation of polyatomic molecules

Results of investigations of the process of multiphoton excitation of polyatomic molecules by CO₂-laser radiation are presented. The mechanism of formation of the profiles of IR absorption bands of polyatomic molecules is discussed. New experimental methods of investigation of relaxation processes at high levels of vibrational excitation of molecules in the ground and triplet states are considered. For vapors of polyatomic molecules and their mixtures with foreign gases, the quantitative characteristics of the collisional exchange and the vibrational-energy transfer as well as the rates of intercombinational conversion ⇛ and triplet-triplet transfer are presented and their dependences on the vibrational-excitation level are discussed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 675–693, September–October, 1998.     

The vibrational quasi-continuum in unimolecular multiphoton dissociation

Summary The vibrational quasi-continuum of the boron trifluoride molecule has been qualitatively studied and the formalism extended to treatN-normal-mode molecules. The anharmonic potential curves for the BF₃ normal modes have been calculated, and the computed anharmonicity constants have been tested against the fundamental frequencies. The potential curve of the wagging mode has been simulated by an internal rotation of one of the fluoride atoms. The vibrational-energy levels and wave functions have been calculated applying second-order perturbation theory. The quasi-continuum energy levels of BF₃ have been obtained by means of a method based in forming adequate linear combinations of wave functions belonging to theN-1 modes resulting from removing the i.r.-active mode; the associated energies have been minimized using a constrained minimization procedure. It has been found that the energy pattern of theN-1 vibrational modes possesses an energy density high enough for constituting a vibrational heat bath and, finally, it has been verified that the ?fictitious? pattern of the active mode is included in the pattern of theN-1 modes.

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Riassunto è stato studiato il quasi-continuo vibrazionale della molecola del trifluoruro di boro e il formalismo è stato esteso per trattare molecola conN modi normali. Sono state calcolate le curve anarmoniche del potenziale per i modi normali di BF₃ e sono state provate le costanti di anarmonicità calcolate rispetto alle frequenze fondamentali. La curva del potenziale del modo di scuotimento è stata simulata mediante una rotazione interna di uno degli atomi del fluoruro. Sono stati calcolati i livelli di energia vibrazionale e le funzioni d'onda, applicando la teoria di perturbazione di secondo ordine. Sono stati ottenuti i livelli di energia quasi continua del BF₃ mediante un metodo basato sulla formazione di adeguate combinazioni lineari di funzione d'onda che appartengono ai modiN-1 che risultano rimuovendo il modo i.r. attivo; le energie associate sono state minimizzate usando una procedura di minimizzazione vincolata. Si è trovato che il modello di energia degliN-1 modi vibrazionali una densità di energia alta abbastanza per costituire un bagno vibrazionale di calore e, infine, si è verificato che il modello ?fittizio? del modo attivo è incluso nel modello degliN-1 modi.

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Резюме Качественно исследуются колебательньій квазиконтиуум молекульі BF₃ и предлагается обобщенньій формализм для рассмотрения молекул сN-нормальньіми модами. Вьічисляются потенциальньіе кривьіе для нормальньіх мод BF₃. Вьічисленньіе константьі ангармоничности проверяются на фундаментальньіх частотах. Потенциальная кривая для модьі моделируется с помощью внутреннего вращения одного из атомов фтора. Вьічисляются уровни знергии колебательного движения и волновьіе функции, используя теорию возмущений второго порядка. Уровни энергии квазиконтинуума BF₃ получаются с помощью метода, основанного на образовании адекватньіх линейньіх комбинаций волновьіх функций, принадлежащихN-1, модам, которьіе возникают при удалении инфракрасной активной модьі. Ассоциированньіе энергии минимизируются, используя ограниченную процедуру минимизации. Получено, что спектр энергии дляN-1 колебательной модьі обладает достаточно вьісокой плотностью для образования колебательной тепловой бани. В заключение, проверяется, что ?фиктивньій? спектр активной модьі включается в спектр дляN-1 модьі.

     

Intracavity system design for IR multiphoton dissociation

An intracavity system for the infrared multiple photon dissociation (IRMPD) of molecules with high dissociation energy threshold has been designed and implemented. The system design based on a TEA CO₂ laser with a cavity folded in V-shape included the analysis of its stability varying the cavity dimensions as well as the analysis of the positions of the beam waists and of the beam size at them. The intracavity energy as a function of the total sample pressure has been measured and the laser-operation threshold has been determined. Intracavity IRMPD has been compared to traditional IRMPD performed in an irradiation geometry in which the radiation is focused into a photoreactor placed outside the laser cavity. Dissociation volumes in intracavity irradiation have resulted an order of magnitude larger than those obtained in experiments performed with the photoreactor outside the laser cavity.     

The dressing effect in multiphoton unimolecular dissociation

Summary On the basis of a quantum-statistical model recently discussed, we deal in this paper with the perturbations induced by the intense field of a CO₂ laser on the levels of the vibrational pattern of a molecule undergoing multiphoton unimolecular dissociation. This perturbational correction is investigated by using a displacement operator technique and the results are interpreted according to the statistical model. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Isotopically selective IR multiphoton dissociation of 1,3,5-trioxane

Searching for oxygen-isotope selectivity, we investigated the infrared multiphoton dissociation of the formaldehyde trimer around 10 µm, in a range where the molecule has a degenerate and a non-degenerate CO stretch vibration and a degenerate CH₂ deformation vibration. In the region of the two latter, the wavelength dependence of the dissociation yield exhibits sharp structures. They were assigned ton-photon resonances (n = 2, 3, 5) by the help of the IR spectrum in the fundamental and overtone region. The O and C selectivities were very small (1.05) near the non-degenerate CO stretch band, but surprisingly large (2–4) in the CH₂ deformation, which has no isotopic shift. The selectivity is not controlled by the first (n-photon) excitation step, but only by a later step. Its assignment is attempted. The pressure dependence of the dissociation is peculiar: Only after an initial decrease, the yield exhibits the usual rise with pressure. We attribute the decrease to a relaxation which perturbs the two-photon resonance.     

甲胺分子的紫外光解离动力学实验研究

在280—287.5 nm区域内,通过实验测定共振增强多光子电离-时间飞行质谱、碎片离子的分质量激发谱以及光强指数等对甲胺分子的光解离通道进行了研究.实验结果证实甲胺分子在单光子能量范围内存在一个电子排斥态,主要的光解离过程为甲胺分子共振吸收1个光子到达该电子排斥态后解离成中性碎片,然后是中性碎片经多光子共振电离形成碎片离子和碎片离子的进一步解离.     

Aromatic C-H bond activation revealed by infrared multiphoton dissociation spectroscopy

Metal-oxide cations are models of catalyst mediating the C-H bond activation of organic substrates. One of the most powerful reagents suggested in the gas phase is based on CuO(+) . Here, we describe the activation of the aromatic C-H bonds of phenanthroline in its complex with CuO(+) . The reaction sequence starts with a hydrogen atom abstraction by the oxygen atom from the 2-position of the phenanthroline ring, followed by OH migration to the ring. Using infrared multiphoton spectroscopy, it is shown that the reaction can be energetically facilitated by additional coordination of a water ligand to the copper ion. As the reaction is intramolecular, a spectroscopic characterization of the product is mandatory in order to unambiguously address the reaction mechanism.     

Pulse duration and collision effects in the multiphoton dissociation of HDCO

Measurements of multiphoton dissociation (MPD) of HDCO gas irradiated by intense CO₂ laser pulses varying in shape and having a duration from 150 ns to ～ 1 μs are reported. It is found that for the same laser pulse energy the short pulses dissociated about an order of magnitude more molecules than the long pulses. Taking into consideration the collision-induced MPD of HDCO previously found, it is shown that to a reasonably good approximation, all the present MPD data for the different pulse durations can be represented on a single, smooth and sharply rising curve of yield versus intensity. This finding is in contrast to the flat intensity response at constant pulse energy found recently for collisionless MPD in SF₆.     

Oxygen isotope separation utilizing two-frequency infrared multiphoton dissociation of 2,3-dihydropyran

Oxygen isotope separation has been examined by utilizing the two-frequency infrared multiphoton dissociation (IRMPD) of 2,3-dihydropyran (DHP). The two-frequency IRMPD reduces the required laser fluences to those lower than the damage threshold of optical windows. For example, dissociation probability of DHP containing ¹⁸O (D(¹⁸O)) and enrichment factor (S) were obtained to be 1.6×10⁻³/pulse and 316, respectively, by the simultaneous irradiation with 1052.2 cm⁻¹ photons at 0.45 J/cm² and 1031.5 cm⁻¹ photons at 1.06 J/cm². These are comparable with D(¹⁸O)=2.2×10⁻³/pulse and S=391 obtained by the single-frequency irradiation of 1033.5 cm⁻¹ photons at 2.2 J/cm². Therefore, the production rate of an ¹⁸O enriched dissociation product has been increased to four times or more, compared with the single-frequency IRMPD, and this two-frequency method would promise a practical large scale separation.     

Infrared laser chemistry and the dynamics of molecular multiphoton excitation

Recent progress in IR-laser chemistry is reviewed. Particular emphasis is placed upon the understanding of the photophysical primary processes of IR-multiphoton excitation in laser chemistry. The available experimental evidence on the main mechanisms of these processes is summarized and compared to various theoretical views. The connection to results on intramolecular rovibrational dynamics from high resolution spectroscopy is established. Some recent quantitative rate data are reviewed in relation to simple theoretical estimates. Several applications of IR-laser chemistry are mentioned, including laser isotope separation and mode selective chemistry. We conclude with a particularly striking potential application of laser chemistry to fundamental aspects of physics, the determination of the parity violating weak nuclear force in polyatomic molecules.