Pressure and intensity dependence of multiphoton energy deposition and reaction yield in vinyl chloride

The frequency, power, and pressure dependence of infrared multiphoton absorption in vinyl chloride has been investigated. The effect of collisions is to allow more energy to be coupled into the higher-pressure samples, which causes the yield of the laser-induced HCl elimination reaction to increase with vinyl chloride pressure.     

Recoil anisotropy following multiphoton dissociation via near-resonant intermediate states

Expressions are derived for the moments beta(n) of the anisotropy of fragment recoil following multiphoton dissociation. The results for all possible two- and three-photon excitation pathways from a sigma or pi initial state, with prompt dissociation, are listed in tabular and graphical form, thereby facilitating the identification of pathways compatible with experimental data. The consequence of possible interference between multiple-excitation pathways is discussed. In addition, allowance is made for rotation of the parent molecule in the dissociating state on the time scale of its dissociation, with specific equations for three cases: fully resolved rotation on excitation, significant but unresolved rotation, and the intermediate case of partially resolved excitation. Finally, expressions are given for the consequence of significant rotation in a near-resonant intermediate state. It is pointed out that the effects of rotation can lead to a strong variation of the anisotropic moments over an excitation band, and that the absence of rotational features does not necessarily imply that the effects of rotation on recoil anisotropy can be ignored. The application of the theory is illustrated for a number of model systems, restricted to diatomic molecules with possible rotation but subsequent axial recoil. However, the expressions are easily adapted to polyatomic systems, provided that any fragment rotation is unresolved.     

Infrared multiphoton dissociation of tetramethoxygermane

Infrared multiphoton dissociation of Ge(OCH₃)₄ molecules by irradiation with a pulsed TEA CO₂ laser has been studied. The basic characteristics of the process have been experimentally investigated. The spectral characterization of the dissociation has been made. The product composition of the dissociation has been analyzed, and possible dissociation pathways have been proposed.     

Infrared multiphoton dissociation of methyl nitrite

Methyl nitrite undergoes dissociation when irradiated with the focused output of a pulsed CO₂ TEA laser. Time resolved infrared emission is observed and attributed to vibrationally excited formaldehyde and methanol. These species are produced via reactions of methoxy radicals formed in the primary dissociation. Other products formed are NO and N₂O.     

Infrared multiphoton dissociation of CHFCl2

We have studied the dependence of the IR laser induced multiphoton dissociation of CHFCl₂ on laser wavelength and fluence, number of laser pulses and initial CHFCl₂ pressure. Multiphoton dissociation spectrum presents a broad maximum centered at 9.342 μm. Dissociation has a strongly dependence on initial CHFCl₂ pressure and laser fluence. There exists a pressure lower limit (≈ 0.4 mb) below which no significant dissociation occurs. Up to 2.1 J/cm² no unimolecular dissociation takes place in a significant proportion. The only radical we have been able to clearly detect by real time optical absorption technique is CFCl. This supports the mechanism reaction CHFCl₂ + nhν → CFCl + HCl.     

Infrared-enhanced multiphoton dissociation of iodobenzene cations

When gas-phase iodobenzene cations in an ion cyclotron resonance spectrometer are irradiated simultaneously with visible 610 nm and IR 977 cm⁻¹ photons, the visible two-photon process is enhanced by a factor of up to three. The experimental pressure and intensity dependences can be modeled using a set of rate equations for the evolution of the populations of different internal energy levels. Modeling suggests that the major mechanism for the mixed visible/IR process is the absorption of several IR photons, followed by the absorption of one visible photon which brings about dissociation: that the visible photoexcitation cross section decreases with increasing ion internal energy: that IR pumping does not compete successfully with relaxation processes near dissociation threshold: and that the IR radiative relaxation time is constant ≈ 500 ms.     

Electron capture dissociation and infrared multiphoton dissociation of oligodeoxynucleotide dications

We report electron capture dissociation (ECD) and infrared multiphoton dissociation (IRMPD) of doubly protonated and protonated/alkali metal ionized oligodeoxynucleotides. Mass spectra following ECD of the homodeoxynucleotides polydC, polydG, and polydA contain w or d "sequence" ions. For polydC and polydA, the observed fragments are even-electron ions, whereas radical w/d ions are observed for polydG. Base loss is seen for polydG and polydA but is a minor fragmentation pathway in ECD of polydC. We also observe fragment ions corresponding to w/d plus water in the spectra of polydC and d(GCATGC). Although the structure of these ions is not clear, they are suggested to proceed through a pentavalent phosphorane intermediate. The major fragment in ECD of d(GCATGC) is a d ion. Radical a- or z-type fragment ions are observed in most cases. IRMPD primarily results in base loss, but backbone fragmentation is also observed. IRMPD provides more sequence information than ECD, but the spectra are more complex due to extensive base and water losses. It is proposed that the smaller degree of sequence coverage in ECD, with fragmentation mostly occurring close to the ends of the molecules, is a consequence of a mechanism in which the electron is captured at a P=O bond, resulting in a negatively charged phosphate group. Consequently, at least two protons (or alkali metal cations) must be present to observe a w or d fragment ion, a requirement that is less likely for small fragments.     

Time dependence of dissociation in the excited state of β-naphthol

Fluorescence decay measurements were used to investigate the mechanism of the acid dissociation of β-naphthol in the excited singlet state. The solutions of the differential equations involved show that the naphthol fluorescence should decay exponentially while the naphtholate ion decay should involve a difference of exponentials. Comparison between the calculated and the measured decay functions indicates that about 20% of the acid dissociation takes place before the thermalized equilibrium excited state, ¹S, is reached.     

Xenon halide exciplex formation by 193 nm laser multiphoton dissociation of vinyl halides in the presence of Xe

Emission from the XeCl(B) and XeBr(B) exciplexes is obtained on ArF laser two-photon excitation of vinyl chloride and vinyl bromide, respectively, in the presence of Xe. The excited XeX(B) states are produced by efficient reactive collisions between the excited B¹Σ⁺ state of HX and ground-state Xe.     

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.     

Mechanism of resonant multiphoton ionization dissociation of p-xylene

The mechanism of resonant multiphoton ionization dissociation (RMPID) of p-xylene is investigated theoretically based both on the quantum-mechanical MO calculation of the geometrical structures and dissociation energies of the fragments and on the computation of laser power-dependent mass spectra of the fragments produced by RMPID. The geometries and dissociation energies are calculated by using both the MNDO method and the ab initio method with the 6-31G basis set. The computation of the mass spectra is carried out in the absorption multiple fragmentation model. It is shown theoretically that the two-independent reaction sequence mechanism proposed by Takenoshita et al. upon measuring the laser power dependence can explain semi-quantitatively the mass spectra of the RMPID. From comparison of the measured mass spectra with the calculated ones it is suggested that the absorption multiple fragmentation model originally based on the product phase space theory can be applied to the RMPID involving transition states in the course of reactions such as the retro-Diels-Alder reaction C₅H⁺₅ → C₃H⁺₃ + C₂H₂ by taking into account the corresponding activation energy instead of the dissociation energy.     

Isotopically selective infrared multiphoton dissociation of 2,3-dihydropyran

Oxygen isotopic selectivity on infrared multiphoton dissociation of 2,3-dihydropyran has been studied by the examination of the effects of excitation frequency, laser fluence, and gas pressure on the dissociation probability of 2,3-dihydropyran and isotopic composition of products. Oxygen-18 was enriched in a dissociation product: 2-propenal. The enrichment factor of 18O and the dissociation probability were measured at a laser frequency between 1033.5 and 1057.3 cm-1, the laser fluence of 2.2-2.3 J/cm2, and the 2,3-dihydropyran pressure of 0.27 kPa. The dissociation probability decreases as the laser frequency being detuned from the absorption peak of 2,3-dihydropyran around 1081 cm-1. On the other hand, the enrichment factor increases with detuning the frequency. The enrichment factor of 18O increases with increasing the 2,3-dihydropyran pressure at the laser fluence of 2.7 J/cm2 or less and the laser frequency of 1033.5 cm-1, whereas the yield of 2-propenal decreases with increasing the pressure. A very high enrichment factor of 751 was obtained by the irradiation of 0.53 kPa of 2,3-dihydropyran at 2.1 J/cm2. Collisional effect of vibrationally excited molecules with ambient molecules on isotopic selectivity is discussed on the basis of a rate equation model including a collisional vibrational de-excitation process.     

Semiclassical formulation of the unimolecular multiphoton dissociation. Dressing of energy levels of the active mode.

Using the minimum coupling Hamiltonian for deascribing the system active mode-laser fields, we have calculated the shifts of the energy levels of the active mode, produced by thr intense laser fields, by means of a methods upon a space-translation transformation. The laser field has been considered classically.     

Classical dynamics of multiphoton dissociation for a model system

A classical trajectory analysis of multiphoton dissociation on a model two-dimensional anharmonic potential surface is presented. Six categories of trajectory motion were found, and the degree of “instability” within each type was analyzed in terms of power spectra and exponential separation of neighboring phase points. The dissociation probability was studied as a function of laser intensity and frequency. In addition, the translational energy spectrum of the photofragments was analyzed on the basis of several statistical theories. Good agreement with RRKM theory was found for the final partitioning between translational and vibrational energy.     

Monochromatic two beam multiphoton dissociation of CF2HCl

Infrared multiphoton dissociation induced by monochromatic two-beam excitation has been carried out by controlling the time delay between the two laser beams. The maximum dissociation yield is found to occur at time delays around 5 μs, when the absorbed energy becomes largely stochastized and an equilibrium among the distinct processes of energy relaxation is manifested. It is shown that this behaviour essentially differs from that which occurs in dichromatic multiphoton dissociation.     

Real-time multiphoton ionization detection of iodine atoms produced by infrared multiphoton dissociation of perfluoroalkyl iodides

We report the direct detection of iodine atoms following infrared multiphoton dissociation of perfluoroalkyl iodides. The technique, three-photon resonant two-photon ionization, shows great promise as an actinometer for primary dissociation yield in IRMPD.     

Infrared multiphoton dissociation calculation of a model linear triatomic

In a previous paper, we reported preliminary results on the multiphoton dissociation of a linear triatomic molecule. This model consists of a dissociative mode (ν₃) coupled non-linearly to an IR inactive harmonic mode (ν₁). We present here extensive calculations of the dissociation probability as a function of the laser frequency for different pulses of constant fluence. It is shown that dissociation occurs at frequencies either very red-shifted from the ν₃ IR active absorption band or located at the ν₂ and ν₃ bands (due to a 2:1 Fermi resonance). A Hose—Taylor analysis reveals that in the former case excitation proceeds through an anharmonic ladder, while a harmonic one is used in the latter case. In both cases essentially Q states are populated during the excitation process. The dissociation process has been dealt with explicitly by using metastable states to represent the continuum. It is shown that the actual structure of the continuum, due to the presence of Feshbach resonant states, has no real influence on the dissociation probability. Fragment analysis for the ABC → ^nhw A + BC dissociation process has been performed and shows only a slight departure from statistical distributions, except at very high intensities.     

Infrared multiphoton dissociation of small-interfering RNA anions and cations

Infrared multiphoton dissociation (IRMPD) on a linear ion trap mass spectrometer is applied for the sequencing of small interfering RNA (siRNA). Both single-strand siRNAs and duplex siRNA were characterized by IRMPD, and the results were compared with that obtained by traditional ion trap-based collision induced dissociation (CID). The single-strand siRNA anions were observed to dissociate via cleavage of the 5′ P—O bonds yielding c- and y-type product ions as well as undergo neutral base loss. Full sequence coverage of the siRNA anions was obtained by both IRMPD and CID. While the CID mass spectra were dominated by base loss ions, accounting for ∼25% to 40% of the product ion current, these ions were eliminated through secondary dissociation by increasing the irradiation time in the IRMPD mass spectra to produce higher abundances of informative sequence ions. With longer irradiation times, however, internal ions corresponding to cleavage of two 5′ P—O bonds began to populate the product ion mass spectra as well as higher abundances of [a − Base] and w-type ions. IRMPD of siRNA cations predominantly produced c- and y-type ions with minimal contributions of [a − Base] and w-type ions to the product ion current; the presence of only two complementary series of product ions in the IRMPD mass spectra simplified spectral interpretation. In addition, IRMPD produced high abundances of protonated nucleobases, [G + H]⁺, [A + H]⁺, and [C + H]⁺, which were not detected in the CID mass spectra due to the low-mass cut-off associated with conventional CID in ion traps. CID and IRMPD using short irradiation times of duplex siRNA resulted in strand separation, similar to the dissociation trends observed for duplex DNA. With longer irradiation times, however, the individual single-strands underwent secondary dissociation to yield informative sequence ions not obtained by CID.     

Infrared multiphoton dissociation of CF3Br

The CF₂, Br, and F products of 9.2 μ multiphoton dissociation of CF₃Br have been spectroscopically monitored. Primary dissociation leads to CF₃ + Br, while secondary dissociation of CF₃ leads to CF₂ + F. Both the Br and F atoms are found to have average recoil energies close to those of thermal (300 K) atoms. Comparison of these results to calculations based on RRKM theory indicates that CF₃Br dissociates from levels 2–5 photons above the CF₃ + Br threshold.