Infrared multiphoton excitation: Energy distribution of reacting molecules from overtone excitation studies

A method is offered to estimate quantitatively the energy distribution of molecules reacting after being excited by multiphoton absorption of infrared radiation. The method is based on the comparison of the real time reaction kinetics obtained in the infrared multiphoton experiment with those observed upon excitation of a vibrational overtone. Results obtained for the dissociation of tetramethyldioxetane under low-pressure conditions are used as a demonstration.     

Infrared multiphoton excitation of refractory metal clusters

Photoexcitation and photoionization experiments on small Tungsten and Niobium clusters were performed with a Q-switched Nd:YAG laser (1064 nm) and a reflectron type time-of-flight mass spectrometer. For low laser fluences the monomer and very small clusters do not show up in the mass spectra. Furthermore, the detected cluster ions show very asymmetric peak shapes caused by delayed ionization (thermionic emission). For high photon fluences photoions with up to charge state +3 could be detected.     

Infrared multiphoton excitation of SO2 to fluorescent states

SO₂ molecules excited under essentially collision-free conditions by intense (>20 G/Wcm²) CO₂ 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.     

Lie algebraic approach to multiphoton excitation of diatomic molecules in intense laser fields

The quadratic anharmonic oscillator Lie algebraic model is used to study the multiphoton transition of the diatomic molecule placed in intense laser fields. The multiphoton excitation of vibration and vibration‐rotation of diatomic molecules in intense laser fields are discussed. In the pure vibration transition we calculate the transition probability versus the frequency of the laser fields for the CO molecule. We also investigate the roles of rotational motion in multiphoton processes and compare with pure vibration for the LiH molecule. The influences of the angular quantum number l and the molecular orientations in laser fields on the multiphoton processes are discussed. The averaged absorb energy changing with the laser field's frequency is calculated. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 201–207, 1999     

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.     

The molecular dynamics problem in multiphoton excitation

While the molecular dynamics for weakly coupled, harmonic oscillators undergoing infinitesimal amplitude displacements are well described by normal modes, and the other extreme — strongly coupled anharmonic, large amplitude oscillating units can be treated as though they are ergodic systems, so that only state densities are needed, intermediate cases are very difficult to analyze. Here we propose that some intermediate cases, wherein we seek relative rates for reaction induced by specific frequency excitation (three-center displacement reactions or dissociation), may be approximately estimated from a normal mode analysis by calculating increment in root mean square amplitudes for a suitable internal coordinate, upon specific multiple photon absorption. A test case is presented as an illustration.     

Luminescence of alkali halide crystals by multiphoton excitation

Luminescence is observed for CsI and NaBr crystals due to two-photon excitation. Near UV fluorescence of a NaI crystal from three-photon excitation is reported. The mechanism of producing near UV luminescence is discussed.     

Continuous wave-based multiphoton excitation fluorescence for capillary electrophoresis

It was reported that a novel detection method, continuous wave (CW)-based multiphoton excitation (MPE) fluorescence detection with diode laser (DL), has been firstly proposed for capillary electrophoresis (CE). Special design of end-column detection configuration proved to be superior to on-column type, considering the detection sensitivity. Three different kinds of fluorescent tags that were widely used as molecular label in bio-analysis, such as small-molecule dye, fluorescent protein and nano particle or also referred to as quantum dot (QD), have been evaluated as samples for the constructed detection scheme. Quantitative analyses were also performed using rhodamine species as tests, which revealed dynamic linear range over two orders of magnitude, with detection limit down to zeptomole-level. Simultaneous detection of fluorescent dyestuffs with divergent excitation and emission wavelengths in a broad range showed advantage of this scheme over conventional laser-induced fluorescence (LIF) detection. Further investigations on CW-MPE fluorescence detection with diode laser for capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC) separations of fluorescein isothiocyanate (FITC) labeled amino acids indicated good prospect of this detection approach in various micro or nano-column liquid phase separation technologies.     

Localization function study of excitation processes in a set of small isoelectronic molecules

Electron localization function (ELF) theory is used to characterize changes that occur upon excitation from ground singlet to first excited triplet states in a series of isoelectronic 16‐electron molecules including H₂CCH₂, HNCH₂, H₂CO, HNNH, HNO, and O₂ (ground triplet to excited singlet). ELF allows one to visualize lone pair or nonbonding electrons, and in these cases the π→π* or n→π excitation processes involved lead to an effective 90° rotation of the electronic structure about one heavy atom center and consequent distortion towards pyramidal symmetry about both heavy atom centers. The heavy atom bond lengths change very little in those cases where effectively two‐center three‐electron bonds can be formed (HNNH, HNO, and O₂) while a significant lengthening occurs in those cases where hydrogen atoms prevent such interactions (H₂CCH₂, HNCH₂, and H₂CO). It is shown that both ELF basin populations and atoms‐in‐molecules (AIM) delocalization indices reflect expected bond orders for conventional single and double bonds provided one compares the ratio of the molecular quantities rather than their absolute magnitudes. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1702–1711, 2001     

N-level Li2 multiphoton rotational wave packets: alignment effects in resonant multiphoton coherent excitation

Using one color ultrafast pump-probe spectroscopy, the authors create N-level multiphoton rotational wave packets via resonant optical pumping between the A((1)Sigma(u) (+)) and E((1)Sigma(g) (+)) electronically bound states of Li(2) from a single optically state-selected rovibrational state |nu(A)=11, j(A)=28>. The authors find that excitation with a single amplitude shaped femtosecond pulse allows the direct observation of up to a six photon absorption, which generates a coherent superposition of 13 rotational states. The multilevel rotational wave packet is theoretically treated with the multipole moment formalism in order to characterize the experimentally observed time-dependent alignment. In particular, the authors find that the magnetic state distributions measured among coherently excited rotational states generated by the resonant multiphoton pumping reduces the measured coherence amplitudes by as much as 40%.     

Infrared multiphoton dissociation with one and two lasers

The dissociation of a diatomic molecule in the presence of one and two infrared lasers is studied. Classical mechanics is used to show that a diatomic molecule may be dissociated much more easily with two laser frequencies than with just one. A short discussion of overlap of resonance follows.     

Time-dependent self-consistent field approach to infrared laser multiphoton excitation

A classical version of the time-dependent self-consistent field (TDSCF) approach is presented for the non-perturbative treatment of intense field multiphoton excitation (MPE) and dissociation (MPD) of polyatomic molecules. The method is applied to the study of infrared MPE of SO₂ It is found that besides being much more computationally efficient, the TDSCF method reproduces the fully classical trajectory results quantitatively. Furthermore, the essential frequency- and intensity-dependent dynamical excitation phenomena predicted by the TDSCF calculations accord with both the experimental and the quantum-mechanical results.     

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 decomposition of highly excited t-butyl chloride-d9

The infrared multiphoton decomposition (IR MPD) of t-butyl chloride-d₉ as a function of pressure and fluence is reported. The major decomposition route was molecular elimination of DCl in accord with earlier pyrolysis studies. The effect of added gases (2-methyl propene-d₈, DCl and 2-methyl propene-h₈) on the MPD is presented. This study is a further example of the utility of a new method of analysis of MPD experiments. © 1994 John Wiley & Sons, Inc.     

Inclusion of discrete-to-continuum coupling in multiphoton excitation and dissociation calculations

A nonperturbative approach to multiphoton excitation and dissociation of molecules is presented in which coupling to the continuum is treated explicitly. Transitions among continuum levels are not modeled directly, but something of their effect is represented by assuming that the continuum population density is so low as to be effectively zero at all times. Two trial applications are briefly discussed.     

Collisionless infrared multiphoton production of electronically excited parent molecules

In the absence of collisions chromyl chloride (CrO₂Cl₂) shows orange fluorescence when the parent molecule is irradiated by the focused output Time-resolved fluorescence is investigated over the pressure range 5 × 10^?4 to 15 torr. As the pressure is increased, a secondary collision-ind We interpret the collisionless part of the emission to arise from infrared transitions between the high vibrational levels of the ground state and the     

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.     

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.