Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy.

Nonlinear optical frequency conversion is one of the most versatile methods to generate wavelength-tunable laser light in the mid-infrared region. This spectral region is particularly important for trace gas detection and other applications of molecular spectroscopy, because it accommodates the fundamental vibrational bands of several interesting molecules. In this article, we review the progress of the most significant nonlinear optics instruments for widely tunable, high-resolution mid-infrared spectroscopy: continuous-wave optical parametric oscillators and difference frequency generators. We extend our discussion to mid-infrared optical frequency combs, which are becoming increasingly important spectroscopic tools, owing to their capability of highly sensitive and selective parallel detection of several molecular species. To illustrate the potential and limitations of mid-infrared sources based on nonlinear optics, we also review typical uses of these instruments in both applied and fundamental spectroscopy.

Double resonant absorption measurement of acetylene symmetric vibrational states probed with cavity ring down spectroscopy.

A novel mid-infrared/near-infrared double resonant absorption setup for studying infrared-inactive vibrational states is presented. A strong vibrational transition in the mid-infrared region is excited using an idler beam from a singly resonant continuous-wave optical parametric oscillator, to populate an intermediate vibrational state. High output power of the optical parametric oscillator and the strength of the mid-infrared transition result in efficient population transfer to the intermediate state, which allows measuring secondary transitions from this state with a high signal-to-noise ratio. A secondary, near-infrared transition from the intermediate state is probed using cavity ring-down spectroscopy, which provides high sensitivity in this wavelength region. Due to the narrow linewidths of the excitation sources, the rovibrational lines of the secondary transition are measured with sub-Doppler resolution. The setup is used to access a previously unreported symmetric vibrational state of acetylene, ν1+ν2+ν3+ν4 (1)+ν5 (-1) in the normal mode notation. Single-photon transitions to this state from the vibrational ground state are forbidden. Ten lines of the newly measured state are observed and fitted with the linear least-squares method to extract the band parameters. The vibrational term value was measured to be at 9775.0018(45) cm(-1), the rotational parameter B was 1.162 222(37) cm(-1), and the quartic centrifugal distortion parameter D was 3.998(62) × 10(-6) cm(-1), where the numbers in the parenthesis are one-standard errors in the least significant digits.

Ionization of acids on the quasi-liquid layer of ice.

The ice quasi-liquid layer (QLL) forms on ice surfaces below the bulk ice melting temperature. It is abundant in the atmosphere, and its importance for atmospheric chemistry is recognized. In the present work, we have studied the microscopic mechanisms of acid ionization on the QLL using ab initio molecular dynamics. The model system QLL is established by nanosecond time scale simulations with empirical force fields, while the reactivity of the QLL is studied using ab initio molecular dynamics. Our ab initio simulations reveal that QLL is reactive, exhibiting stable crystalline point defects, which contribute to efficient acid solvation, ionization, and proton transfer. We study in detail deuterated hydrogen iodide (DI) and nitric acid (DNO3). Ionization in both cases benefits from the abundance of weakly bonded hydrogen-bond single-acceptor double-donor water molecular species available on the QLL in high relative concentration. Picosecond time scale ionization is demonstrated for both molecular species. Our results suggest efficient reactivity of acid ionization and proton transfer at temperature ranges appropriate for the upper troposphere and lower stratosphere.

(H2O)20 water clusters at finite temperatures.

We have performed an exhaustive study of energetics of (H2O)20 clusters. Our goal is to study the role that various free-energy terms play in this popular model system and see their effects on the distribution of the (H2O)20 clusters and in the infrared spectrum at finite temperatures. In more detail, we have studied the electronic ground-state structure energy and its long-range correlation (dispersion) part, vibrational zero-point corrections, vibrational entropy, and proton configurational entropy. Our results indicate a delicate competition between the energy terms; polyhedral water clusters are destabilized by dispersion interaction, while vibrational terms (zero-point and entropic) together with proton disorder entropy favor them against compact structural motifs, such as the pentagonal edge- or face-sharing prisms. Apart from small water clusters, our results can be used to understand the influence of these energy terms in water/ice systems in general. We have also developed energy expressions as a function of both earlier proposed and novel hydrogen-bond connectivity parameters for prismatic water clusters.

Global minima of protonated water clusters (H2O)20H+ revisited.

A graph-theoretical analysis is performed on the (H(2)O)(20) "edge-sharing pentagonal prism" cluster to find proton configurations that yield the lowest cluster total energies. Using the low-energy structures, we create models for protonated (H(2)O)(20)H(+) clusters that compete energetically with "cage-like" structures proposed earlier in the literature. We perform benchmarking between different theoretical methods and observe significant stabilization of compact versus polyhedral clusters due to long-range electron correlation effects, which make the comparison between different cluster morphologies difficult using density functional theory only. All methodologies that we used (up to the second level of Møller-Plesset perturbation theory (MP2)) agree that for protonated clusters, the cage-like morphology proposed in [ Chem. Phys. Lett. 2000, 324, 279-288] is the most stable one. We study in detail several (H(2)O)(20)H(+) cluster structures and suggest that the energetics in small protonated water clusters is dominated by the competition of open polyhedral structures, favored by the Eigen H(9)O(4)(+) species, against van der Waals interaction and "ice rules", which both favor compact structural motifs such as the prismatic particle. We demonstrate this tendency using ab initio calculations for prismatic, dodecahedral, and cage-like clusters, while the "magic number" cluster (H(2)O)(n)H(+), n = 21, is observed to minimize all competing energy contributions simultaneously. We emphasize the utility of the cage-like cluster as a model template for ice-related atmospheric reactions and benchmark the GPAW density functional theory code for making such calculations.

Biochemical pathways of breath ammonia (NH3) generation in patients with end-stage renal disease undergoing hemodialysis.

Breath ammonia (NH3) has been proposed as a potential biomarker in monitoring hemodialysis (HD) adequacy, since a strong correlation between blood urea and mouth-exhaled breath NH3 has been observed in patients with end-stage renal disease (ESRD) undergoing HD. However, the biochemical pathways for breath NH3 generation from blood urea have not been demonstrated. In this study, we show a strong correlation (r s = 0.77, p < 0.001) between blood and salivary urea, indicating that salivary urea levels reflect blood urea levels. Salivary urea is in turn strongly correlated to salivary ammonia ([Formula: see text] + NH3) in most of the patients. This confirms that the hydrolysis of urea by urease generates ammonia in the oral cavity. A further strong correlation between salivary ammonia and breath NH3 indicates that salivary ammonia evaporates into gas phase and turns to breath NH3. Therefore, blood urea is a major biochemical source of breath NH3. Since breath NH3 is generated predominantly in the oral cavity, the levels of breath NH3 are influenced significantly by the patient's oral condition including urease activity and salivary pH. Our results agree with previous studies that have shown a connection between salivary urea and breath NH3.

Configurational Entropy in Ice Nanosystems: Tools for Structure Generation and Screening.

Recently, a number of experimental and theoretical studies of low-temperature ice and water in nanoscale systems have emerged. Any theoretical study trying to model such systems will encounter the proton-disorder problem, i.e., there exist many configurations differing by water-molecule rotations for a fixed oxygen atom structure. An extensive search within the allowed proton-disorder space should always be perfomed to ensure a reasonable low-energy isomer and to address the effect of proton-configurational entropy that may affect experimental observables. In the present work, an efficient general-purpose program for finite, semiperiodic, and periodic systems of hydrogen-bonded molecules is presented, which can be used in searching and enumerating the proton-configurational ensemble. Benchmarking tests are performed for ice nanotubes and finite slabs. Finally, the program is applied to experimentally appropriate ice nanosystems. A boron nitride film supported ice nanodot is studied in detail. Using a systematic generation of its proton-configurational ensemble, we find an isomer that is ∼1 eV lower in total energy than one previously studied. The present isomer features a considerable dipole moment and implies that ice nanodots are inherently ferroelectric parallel to the surface. We conclude by demonstrating how the so-called hydrogen-bond connectivity parameters can be used to screen low-energy isomers.

Hydrogen cyanide in the headspace of oral fluid and in mouth-exhaled breath.

Mouth-exhaled hydrogen cyanide (HCN) concentrations have previously been reported to originate from the oral cavity. However, a direct correlation between the HCN concentration in oral fluid and in mouth-exhaled breath has not been explicitly shown. In this study, we set up a new methodology to simultaneously measure HCN in the headspace of oral fluid and in mouth-exhaled breath. Our results show that there is a statistically significant correlation between stimulated oral fluid HCN and mouth-exhaled HCN (rs = 0.76, p < 0.001). This confirms that oral fluid is the main contributor to mouth-exhaled HCN. Furthermore, we observe that after the application of an oral disinfectant, both the stimulated oral fluid and mouth-exhaled HCN concentrations decrease. This implies that HCN production in the oral cavity is related to the bacterial and/or enzymatic activity.

The origin of mouth-exhaled ammonia.

It is known that the oral cavity is a production site for mouth-exhaled NH3. However, the mechanism of NH3 production in the oral cavity has been unclear. Since bacterial urease in the oral cavity has been found to produce ammonia from oral fluid urea, we hypothesize that oral fluid urea is the origin of mouth-exhaled NH3. Our results show that under certain conditions a strong correlation exists between oral fluid urea and oral fluid ammonia (NH4(+)+NH3) (rs = 0.77, p < 0.001). We also observe a strong correlation between oral fluid NH3 and mouth-exhaled NH3 (rs = 0.81, p < 0.001). We conclude that three main factors affect the mouth-exhaled NH3 concentration: urea concentration, urease activity and oral fluid pH. Bacterial urease catalyses the hydrolysis of oral fluid urea to ammonia (NH4(+)+NH3). Oral fluid ammonia (NH4(+)+NH3) and pH determine the concentration of oral fluid NH3, which evaporates from oral fluid into gas phase and turns to mouth-exhaled NH3.

Ionization of Nitric Acid on Crystalline Ice: The Role of Defects and Collective Proton Movement.

Ionization of nitric acid (HNO3) on a model ice surface is studied using ab initio molecular dynamics at temperatures of 200 and 40 K with a surface slab model that consists of the ideal ice basal plane with locally optimized and annealed defects. Pico- and subpicosecond ionization of nitric acid can be achieved in the defect sites. Key features of the rapid ionization are (a) the efficient solvation of the polyatomic nitrate anion, by stealing hydrogen bonds from the weakened hydrogen bonds at defect sites, (b) formation of contact ion pairs to stable "presolvated" molecular species that are present at the defects,

Ammonia in breath and emitted from skin.

Ammonia concentrations in exhaled breath (eNH3) and skin gas of 20 healthy subjects were measured on-line with a commercial cavity ring-down spectrometer and compared to saliva pH and plasma ammonium ion (NH(+)4), urea and creatinine concentrations. Special attention was given to mouth, nose and skin sampling procedures and the accurate quantification of ammonia in humid gas samples. The obtained median concentrations were 688 parts per billion by volume (ppbv) for mouth-eNH3, 34 ppbv for nose-eNH3, and 21 ppbv for both mouth- and nose-eNH3 after an acidic mouth wash (MW). The median ammonia emission rate from the lower forearm was 0.3 ng cm(-2) min(-1). Statistically significant (p < 0.05) correlations between the breath, skin and plasma ammonia/ammonium concentrations were not found. However, mouth-eNH3 strongly (p < 0.001) correlated with saliva pH. This dependence was also observed in detailed measurements of the diurnal variation and the response of eNH3 to the acidic MW. It is concluded that eNH3 as such does not reflect plasma but saliva and airway mucus NH(+)4 concentrations and is affected by saliva and airway mucus pH. After normalization with saliva pH using the Henderson-Hasselbalch equation, mouth-eNH3 correlated with plasma NH(+)4, which points to saliva and plasma NH(+)4 being linked via hydrolysis of salivary urea.

Background levels and diurnal variations of hydrogen cyanide in breath and emitted from skin.

The hydrogen cyanide (HCN) concentration in exhaled human breath and skin gas samples collected with different sampling techniques was measured using near-infrared cavity ring-down spectroscopy. The median baseline HCN concentrations in samples provided by 19 healthy volunteers 2-4 h after the last meal depended on the employed sampling technique: 6.5 parts per billion by volume (ppbv) in mixed (dead space and end-tidal) mouth-exhaled breath collected to a gas sampling bag, 3.9 ppbv in end-tidal mouth-exhaled breath, 1.3 ppbv in end-tidal nose-exhaled breath, 1.0 ppbv in unwashed skin and 0.6 ppbv in washed skin samples. Diurnal measurements showed that elevated HCN levels are to be expected in mouth-exhaled breath samples after food and drink intake, which suggests HCN generation in the oral cavity. The HCN concentrations in end-tidal nose-exhaled breath and skin gas samples were correlated, and it is concluded that these concentrations best reflect systemic HCN levels.

Acetylene in breath: background levels and real-time elimination kinetics after smoking.

We have measured the acetylene concentration in the exhaled breath of 40 volunteers (31 non-smokers, nine smokers) using near-infrared cavity ring-down spectroscopy. The acetylene levels were found to be the same as in ambient air for non-smokers, whereas elevated levels were observed for smokers. Real-time measurements with sub-second time resolution have been applied to measure the elimination kinetics of acetylene in breath after exposure to tobacco smoke. Three exponential time constants can be distinguished from the data and these can be used to define the residence times for different compartments, according to the multi-compartment model of the human body.

Visual performance in night-time driving conditions.

This paper introduces an experimental multitechnique method which was developed to establish a basis for a task performance-based mesopic photometry. This approach considers night-time driving by dividing visual performance into three visual tasks, of which achromatic threshold and reaction time are presented. The performance of both visual tasks decreased with decreasing luminance level from 1 to 0.01 cd m(-2), showing the strong effect of light level on visual performance in driving. The behaviour of the achromatic contrast threshold and reaction time for low-contrast targets was similar in terms of spectral effects, the strongest effects occurring at the lower mesopic levels. Both measures showed the Purkinje shift with decreasing luminance levels. The experimental data were used to calculate mesopic performance measures with the new mesopic model. The results imply that compared with V(lambda), spectral sensitivity in night-time driving can be better described with a mesopic model based on visual performance measures.

First stretching overtone of BiH3: an extreme local-mode case for XH3-type molecule?

The first stretching overtone region of short-lived, formerly inaccessible BiH3 near 3405 cm(-1) has been measured by Fourier-transform infrared spectroscopy with a resolution of 0.0066 cm(-1). Only the 2nu1(A1)/nu1+nu3(E) band system has been observed. Rotational analysis, with transitions reaching J'max=14, has revealed almost perfect local-mode behavior for the upper states denoted as (200A1/E) in the local-mode notation. Ratios of vibration-rotation interaction parameters q(eff)/alpha(eff)(BB) and r(eff)/alpha(eff)(BC), and the appropriate rotational constant differences, are in good agreement with theoretical local-mode limit values. A simple stretching vibrational model reproduces the observed vibrational term values well, and the potential parameters obtained are close to true values.

Sleep and cardiac rhythm in healthy men.

Cardiac rhythm during sleep was analysed in 168 healthy men aged 19 to 30 (mean 21.8) years using continuous nocturnal ECG recordings. Thirty five of the subjects were endurance athletes; 99 were recorded at home and 69 at a garrison during military service. The number of short term (less than one min) accelerations of heart rate of more than ten beats per minute ranged from 1.3 to 14.3 per hour; the number or accelerations (greater than one min) of more than 25 beats per minute typical of nocturnal restlessness ranged from zero to 6.1 per hour. The accelerations indicating nocturnal restlessness were more common during recordings done at the garrison than at home (2.2 per hour vs 1.3 per hour, P less than 0.01), whereas the short term accelerations were not (5.8 per hour vs 5.1 per hour, NS). Sinus pauses exceeding 2.00 sec occurred in 17 men (13 in athletes), occasional second degree atrioventricular block in 19 (eight in athletes) and ventricular premature beats in 37. Eighty-two per cent of sinus pauses, 83% of second degree atrioventricular blocks, and 81% of ventricular premature beats (when less than ten in the same subject) were associated with short term changes in heart rate. In conclusion, rapid changes in heart rate, presumably as a results of autonomic activation, are closely associated with most arrhythmias in healthy men during sleep, whereas changes in heart rate typical of nocturnal restlessness have little arrhythmogenic effect.

High Resolution FTIR Study of the nu5 Bands of HSiD3 and H120SnD3

High resolution Fourier Transform spectra of HSiD3 and monoisotopic H120SnD3 have been recorded in the region of the nu5 fundamental at 850.68 and 646.90 cm-1 with a resolution of 3.3 and 2.8 x 10(-3) cm-1, respectively. About 2000 rovibrational transitions of each species have been assigned and fitted to two different reductions of the effective Hamiltonian, with sigma(Fit) = 1.2 x 10(-4) (HSiD3) and 1.4 x 10(-4) cm-1 (H120SnD3). The two sets of parameters were shown to be equivalent, and relations between parameters belonging to different reductions are perfectly fulfilled. Furthermore the ground state constants C0 and DK0 have been determined for the first time. For consistency the previously measured data belonging to the nu5 band of H70GeD3 have been refitted, sigma(Fit) = 1.6 x 10(-4) cm-1, on the basis of improved ground state parameters including the h'3 term accounting for the splitting of the K = 3 level. Copyright 1998 Academic Press.

High Resolution Study of the nu2 , 2nu1 , nu1 + nu3 , and 2nu3 Bands of Hydrogen Telluride: Determination of Equilibrium Rotational Constants and Structure

High resolution Fourier transform spectra of a natural and a 130 Te monoisotopic sample of H2 Te have been recorded at a resolution of 0.0022 cm-1 in the 11.6 µm spectral region, as well as a spectrum of a natural sample of H2 Te at a resolution of 0.0051 cm-1 in the 2.4 µm region. In the 11.6 µm region the main absorbing band is the nu2 band, the analysis of which was rather easy. On the other hand, in the 2.4 µm region three bands are absorbing, namely 2nu1 , nu1 + nu3 , and 2nu3 , the last being much weaker than the others. The analysis in this spectral domain was much more difficult because of resonances. Indeed it proved not possible to reproduce the observed lines without taking into account the Darling-Dennison interaction between the levels of the (200) and (002) states and the Coriolis interactions between the levels of (200) and (101) and between those of (101) and (002). Considering these interactions allowed us to calculate very satisfactorily all the experimental levels, and precise sets of vibrational energies and rotational and coupling constants were obtained for the seven most abundant H2 Te isotopic species, namely H2 130 Te, H2 128 Te, H2 126 Te, H2 125 Te, H2 124 Te, H2 123 Te, and H2 122 Te. For the most abundant species, H2 130 Te, the band centers in cm-1 are nu0 (nu2 ) = 860.6563, nu0 (2nu1 ) = 4062.8842, nu0 (nu1 + nu3 ) = 4063.3697, and nu0 (2nu3 ) = 4137.0454. These results, combined with those obtained for other vibrational states, have been used to derive the equilibrium rotational constants and their corrections. Finally, by neglecting the electronic corrections, the equilibrium structure of H2 130 Te was obtained as follows: r e (Te-H) = 1.65145(10) A, alphae (HTeH) = 90.2635(90)degrees.

High-Resolution FTIR and Photoacoustic Overtone Spectrum of HCCI

The first and second C-H overtone vibration-rotation absorption spectra of monoiodoacetylene (HCCI) have been recorded using an FTIR spectrometer with an instrument resolution of about 0.005 cm-1 (0.9/MOPD). The third overtone spectrum has been obtained using a titanium:sapphire ring laser spectrometer with a spectral resolution of about 0.02 cm-1. Due to anharmonic resonances like Fermi and Darling-Dennison resonances, accompanying bands can be observed close to the main overtone band. All the observed bands have been rotationally analyzed. Published vibrational data together with present results are used in a model which includes anharmonic resonances. Vibrational assignments of the observed bands are based on this calculation. The agreement between calculated and observed energy levels is good considering the amount of data available. Copyright 1997 Academic Press. Copyright 1997Academic Press

High-Resolution Infrared Spectrum of nu5, nu4, nu3, nu4 + nu5, and 2nu4 Band Systems of Deuterobromoacetylene.

High-resolution vibration-rotation spectra of gas-phase deuterobromoacetylene have been recorded in the 240-990 cm-1 infrared region. The analyzed band systems are rich in hot bands and have a high density of lines. Five band systems and a total of 124 vibration-rotation bands of the isotopic species DCC79Br and DCC81Br have been rotationally analyzed. Accurate rotational parameters and vibrational wavenumbers for 33 vibrational states of each species have been obtained from the rotational analysis. l doubling and rotational l resonance have been observed on some states and the respective resonance parameters have been obtained through nonlinear least-squares optimization. A Fermi resonance block model with perturbation terms has been used for the analysis of the vibrational states. With optimized parameters, the model produces root-mean-square deviations of observed - calculated wavenumbers of about 0.3 cm-1 for both isotopic species. Copyright 1999 Academic Press.