Complex Molecular Spectra Research Articles

Wave packet dynamics and control in excited states of molecular nitrogen.

Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4-9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.

Photon-counting distributed free-space spectroscopy

Spectroscopy is a well-established nonintrusive tool that has played an important role in identifying and quantifying substances, from quantum descriptions to chemical and biomedical diagnostics. Challenges exist in accurate spectrum analysis in free space, which hinders us from understanding the composition of multiple gases and the chemical processes in the atmosphere. A photon-counting distributed free-space spectroscopy is proposed and demonstrated using lidar technique, incorporating a comb-referenced frequency-scanning laser and a superconducting nanowire single-photon detector. It is suitable for remote spectrum analysis with a range resolution over a wide band. As an example, a continuous field experiment is carried out over 72 h to obtain the spectra of carbon dioxide (CO2) and semi-heavy water (HDO, isotopic water vapor) in 6 km, with a range resolution of 60 m and a time resolution of 10 min. Compared to the methods that obtain only column-integrated spectra over kilometer-scale, the range resolution is improved by 2–3 orders of magnitude in this work. The CO2 and HDO concentrations are retrieved from the spectra acquired with uncertainties as low as ±1.2% and ±14.3%, respectively. This method holds much promise for increasing knowledge of atmospheric environment and chemistry researches, especially in terms of the evolution of complex molecular spectra in open areas.

Comb-resolved spectroscopy with immersion grating in long-wave infrared.

We have developed a dispersive spectrometer by using a compact immersion grating for direct frequency comb spectroscopy in the long-wave infrared region of 8-10 μm for the first time. A frequency resolution of 460 MHz is achieved, which is the highest reported in this wavelength region with a dispersive spectrometer. We also demonstrate individual comb mode-resolved imaging by cavity filtering and apply this to obtain spectra of both simple and complex molecular spectra. These results indicate that the immersion grating spectrometer offers the next advancement for sensitive, high-resolution spectroscopy of transient and large/complex molecules when combined with cavity enhancement and cooling techniques.

Application of the mixed time-averaging semiclassical initial value representation method to complex molecular spectra.

The recently introduced mixed time-averaging semiclassical initial value representation of the molecular dynamics method for spectroscopic calculations [M. Buchholz, F. Grossmann, and M. Ceotto, J. Chem. Phys. 144, 094102 (2016)] is applied to systems with up to 61 dimensions, ruled by a condensed phase Caldeira-Leggett model potential. By calculating the ground state as well as the first few excited states of the system Morse oscillator, changes of both the harmonic frequency and the anharmonicity are determined. The method faithfully reproduces blueshift and redshift effects and the importance of the counter term, as previously suggested by other methods. Different from previous methods, the present semiclassical method does not take advantage of the specific form of the potential and it can represent a practical tool that opens the route to direct ab initio semiclassical simulation of condensed phase systems.

Multiplexed sub-Doppler spectroscopy with an optical frequency comb.

An optical frequency comb generated with an electro-optic phase modulator and a chirped radiofrequency waveform is used to perform pump-probe spectroscopy on the D1 and D2 transitions of atomic potassium at 770.1 nm and 766.7 nm, respectively. With a comb tooth spacing of 200 kHz and an optical bandwidth of 2 GHz the hyperfine transitions can be simultaneously observed. Interferograms are recorded in as little as 5 μs (a timescale corresponding to the inverse of the comb tooth spacing). Importantly, the sub-Doppler features can be measured as long as the laser carrier frequency lies within the Doppler profile, thus removing the need for slow scanning or a priori knowledge of the frequencies of the sub-Doppler features. Sub-Doppler optical frequency comb spectroscopy has the potential to dramatically reduce acquisition times and allow for rapid and accurate assignment of complex molecular and atomic spectra which are presently intractable.

Look, no hands! Spectral biomarkers from genetic association studies

Recent advances in our understanding of the genomics of the human metabolome have shed light on the pathways involved in metabolic and cardiovascular disease. Such studies crucially depend on the interpretation of complex molecular spectra. A recent study by Suhre and colleagues provides a way to identify potentially clinically relevant biomarkers without a priori information, such as reference spectra, thus aiding the discovery of additional spectral features and corresponding genomic loci associated with metabolism and disease.

Saturation effects in molecular spectroscopy using degenerate four‐wave mixing

AbstractLineshapes of degenerate four‐wave mixing signals induced by both monochromatic and non‐monochromatic lasers are compared with predictions of the non‐perturbative model of Bratfalean, Lloyd and Ewart (BLE model) for the interaction of pump and probe beams of arbitrary intensity. Spectral lineshapes of isolated lines in DFWM spectra of OH in a methane–oxygen flame were recorded using a novel single‐mode tunable laser as a function of laser power. Power broadening of the lineshapes was found, in this case of an essentially monochromatic laser, to be well described by the BLE model. DFWM lineshapes of more complex molecular spectra were recorded in C2 in an oxy–acetylene flame using a standard, non‐monochromatic, dye laser. Lineshapes of closely spaced molecular resonances were found to be appreciably affected by saturation or power broadening effects. These effects were reasonably well approximated by the non‐perturbative BLE model when adapted to take account of the finite laser bandwidth. In contrast, the experimental lineshapes of the C2 spectra for saturating fields could not be modeled using the standard perturbative model of Abrams and Lind Copyright © 2003 John Wiley & Sons, Ltd.

Sensitive detection of nitric oxide using seeded parametric four-wave mixing

A sensitive near-resonant four-wave mixing technique based on two-photon parametric four-wave mixing has been developed. Seeded parametric four-wave mixing requires only a single laser as an additional phase matched “seeder” field is generated via parametric four-wave mixing of the pump beam in a high gain cell. The seeder field travels collinearly with the pump beam providing efficient nondegenerate four-wave mixing in a second medium. This simple arrangement facilitates the detection of complex molecular spectra by simply scanning the pump laser. Seeded parametric four-wave mixing is demonstrated in both a low pressure cell and an air/acetylene flame with detection of the two-photon C 2Π(v′=0)←X 2Π(v″=0) spectrum of nitric oxide. From the cell data a detection limit of 1012 molecules/cm3 is established. A theoretical model of seeded parametric four-wave mixing is developed from existing parametric four-wave mixing theory. The addition of the seeder field significantly modifies the parametric four-wave mixing behaviour such that in the small signal regime, the signal intensity can readily be made to scale as the cube of the laser pump power while the density dependence follows a more familiar square law dependence. In general, we find excellent agreement between theory and experiment. Limitations to the process result from an ac Stark shift of the two-photon resonance in the high pressure seeder cell caused by the generation of a strong seeder field, as well as a reduction in phase matching efficiency due to the presence of certain buffer species. Various optimizations are suggested which should overcome these limitations, providing even greater detection sensitivity.

Infrared Seeded Parametric Four-Wave Mixing for Sensitive Detection of Molecules

We have developed a sensitive resonant four-wave mixing technique based on two-photon parametric four-wave mixing with the addition of a phase matched ``seeder'' field. Generation of the seeder field via the same four-wave mixing process in a high pressure cell enables automatic phase matching to be achieved in a low pressure sample cell. This arrangement facilitates sensitive detection of complex molecular spectra by simply tuning the pump laser. We demonstrate the technique with the detection of nitric oxide down to concentrations more than 4 orders of magnitude below the capability of parametric four-wave mixing alone, with an estimated detection threshold of ${10}^{12}\mathrm{molecules}/{\mathrm{cm}}^{3}$.

The phase character of Zeeman modulation magnetic rotation laser spectroscopy

Zeeman modulation-magnetic rotation spectroscopic (ZM-MRS) technique is provided with not only high sensitivity and high signal-to-noise ratio, but also apparent phase character for studying molecular spectrum. It is quite helpful for assigning some complex molecular rotational spectra. As an example, the ZM-MRS spectra of NO2 molecule was measured in the region between 16838cm-1 and 16860cm-1 with high resolution. The semi-classical magnetic rotation theory was used to explain the spectral feature and its phase relation with transition quantum number. It is in agreement with the experimental results. Finally, by using matrix diagonalization of effective Hamiltonian, the ZM-MRS spectra of NO2 molecule were analyzed and a reasonable assignment was obtained.

Collisional Excitation of O +2 and N +2 Electronic States via Single Electron Capture by B 2+ Ions

High-resolution translational energy spectra have been obtained for single-electron capture by 4-keV B 2+ ions in collision with Ne, O 2, and N 2. The Ne spectrum is used for accurate determination of the zero energy-change position for subsequent analysis of the complex molecular spectra. Six capture channels have been unambiguously identified for the O 2 spectrum; two of these involve capture-excitation, which leaves the O + 2 ion in its fourth excited state, responsible for subsequent emission of the first negative band system [ b 4Σ − g → a 4Π u ] of O + 2. The N 2 spectrum consists of seven narrow spectral features, four of which are identified with capture channels from which the first and second excited electronic states of the N + 2 ion are accessed. These constitute the upper states of the Meinel [ A 2Π u → X 2Σ − g ] and first negative [ B 2Σ + u → X 2Σ + g ] band systems of the N + 2 ion.

Resonance coherent anti-Stokes Raman scattering in nitrogen dioxide using a broadband dye laser

We have applied electronically resonant coherent anti-Stokes Raman scattering (CARS) to the detection of nitrogen dioxide. The resonance CARS spectrum of NO2 in a sample cell was recorded around a Raman shift of 1500 cm-1 using a frequency-doubled injection-seeded single-mode Nd:YAG laser and a tunable dye laser. In particular, we demonstrate the use of a broadband Stokes laser to measure resonance CARS spectra of NO2. Recordings at various pump laser frequencies were made by temperature tuning of the Nd:YAG master laser. The pressure dependence of the signal was also investigated. The resonance CARS results are compared with laser-induced fluorescence measurements. The work shows the high resolution achievable in complex molecular spectra using resonance CARS.

X-ray and Auger de-excitation processes in resonantly-excited and core-ionized molecules

Some recent developments in the understanding of molecular Auger and X-ray spectra are reviewed. Factors leading to complex molecular Auger spectra are discussed. The importance of correlation satellites in some spectra, such as for the SiF 4 molecule, are reported. The prediction of inner-shell photoelectron linewidths and the discrepancies with experiment for silicon-containing molecules are reported. The strong dependence of resonant X-ray emission spectra on photon excitation energy is discussed in the context of the CO 2 and N 2O molecules.

Robust Estimation of Term Values in High-Resolution Spectroscopy: Application to the e3Σ +u → a3Σ +g Spectrum of T2

In complex molecular spectra the identification of the pair of states corresponding to an observed transition energy is a difficult process. Erroneous assignments are hard to avoid. They cause fundamental difficulties when using least-squares procedures for estimating term values. On the other hand, a robust fitting method has proven extremely helpful for finding erroneous assignments and for obtaining reliable estimates of term values. These facts are demonstrated by applying both procedures to the estimation of term values in the e 3Σ + u → a 3Σ + g bands of T 2.

Theoretical calculation of the spectral fluctuation of the molecular NO 2 spectrum

In this paper we present a novel theory suitable for the calculation of statistical properties of complex molecular spectra. By using this theory the spectral fluctuations of NO 2 VIS spectra are calculated. The existence of irregular spectra in the NO 2 VIS spectra is confirmed.

Statistical Properties of Energy Levels in Non‐Born‐Oppenheimer Systems

AbstractBasic ideas and methods of the statistical analysis of energy spectra are reviewed. Complex nuclear, atomic and molecular spectra show typical fluctuation patterns which can be modeled with the aid of random matrices. Calculations on model systems and theoretical results show the existence of two universality classes of spectral fluctuations which depend on whether there are strong or only weak couplings between the various degrees of freedom. This correspondence also provides a link between quantum spectral statistics and classical dynamics. We illustrate statistical methods by analyzing model spectra obtained from a Hamiltonian which describes non‐Born‐Oppenheimer coupling of nuclear and electronic motion in molecules.

High resolution molecular spectroscopy with lasers

Several sensitive detection techniques, such as excitation spectroscopy or resonant two-photon ionization spectroscopy are applied to laser spectroscopy with sub-Doppler resolution in collimated molecular beams. Due to internal cooling during the adiabatic expansion in supersonic beams rotational temperatures below 10 K can be reached which results in a drastic simplification of otherwise complex molecular spectra.

Physical structure of cellulose with the help of the programmed usage of molecular spectroscopy methods

Systematic investigations in the area of complex automation of the method of IR spectroscopy have permitted developing an extensive complex of programs and the corresponding instrumentation as well as creating a system for the programmed usage of the IR method of spectroscopy (the PUMIS system). All the programs are written in FORTRAN and are separate modules which operate with one and the same input and output buffer of the operational memory of the computer and transmit the necessary information to each other [5-9]. An arbitrary sequence of execution of these modules is possible, which provides for flexibility of the mathematical apparatus and its application to the most diverse problems. The fundamentally important problems of discretization of infrared spectra by digital analysis of them, the choice of the shape of the profile of absorption bands, and the separation of complex bands into components have been discussed in detail in [14, 15]. Systematic investigations of many years in the area of the spectroscopy of cellulose and model systems [16-19] and the application of a complex of developed systematic methods and programmed analysis of experimental data with the help of computers have permitted first obtaining a set of analytic parameters characterizing the specific properties of the physical structure of cellulose as a function of its origin, means of extraction, and processing (Fig. i, Table i). We shall call this method the multiparametric complex spectroscopic method (MCS). Up to now people have been limited in the investigation of vibrational spectra of this kind of complex compounds as a rule to the analysis of only two or three parameters (frequency at the maximum, peak and integrated intensities of the absorption band). A sufficiently complete extraction of all the information from complex molecular spectra has become possible on the basis of the application of computers and contemporary mathematical methods. This is especially important in the case of compounds with a variety of conformation properties of the macromolecules and intra- and intermolecular interactions and with a complex system of overlapping absorption bands in the spectra. The effectiveness of the application of the automated MCS method to the investigation of cellulose involves the specific properties of the structural properties of this polymer. It is well known that the diversity of properties of the physical structure of cellulose is determined by the possibility of the existence of different combinations of rotamers of the hydroxyl and oxymethyl groups with relatively stable forms of the pyranyl rings and chains of macromolecules. This in turn causes a broad range of energetically nonequivalent hydrogen bonds and intra- and intermoiecular ordering. Precisely these parameters -- the primary source of structural features of cellulose -- are established with the help of the MCS method (difference of the hydrogen bonds in ordered and disordered regions and their multiplicity, rotational isomerism of the hydroxyl and oxymethyl groups, relationship of the ordered and disordered regions, their uniformity and stability, the difference in ordering of the structure on the surface of a sample and in its interior, and so on). Thus, the automated MCS method permits analyzing a sufficiently complete set of important characteristics of the structure of cellulose which determine the specific properties of its physical structure. In our opinion one should isolate especially the parameters which char

A dynamic algebra for rotation-vibration spectra of complex molecules

An algebraic approach to tri- and poly-atomic molecules is presented. This approach is applied to the study of linear triatomic molecules. It is suggested that these techniques may be useful in the description of complex molecular rotation-vibration spectra.

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Reply to comment on “green's functions in the theories of radiationless transitions, complex molecular spectra and resonant Raman cross sections