Hot Band Absorption Research Articles

High resolution Fourier transform infrared spectroscopy of the ν6 and ν10 bands of jet-cooled Fe(CO)5

Rovibrational FTIR spectra of Ni(CO)4 and Fe(CO)5 have been recorded in supersonic argon expansions at resolutions up to 0.004 cm−1. Rotational temperatures as low as 4 K for the ν6 band of Fe(CO)5 and 6 K for the ν5 band of Ni(CO)4 have been determined from the populations of the rotational levels, and an efficient vibrational cooling has been evidenced by the large reduction of hot band absorptions. An analytical model including the instrumental line shapes and the intensity distribution is proposed to enable simulations of overlapping rovibrational transitions of symmetric top molecules. The spectroscopic study of the ν6 and ν10 stretching bands of Fe(CO)5 yielded the following parameters (3σ uncertainties in parenthesis): ν0=2038.106 97(91) cm−1, B6=0.026 797(12) cm−1, and B0=0.026 826(10) cm−1 for the 610 transition, ν0=2015.5513(12) cm−1, ΔA10=−0.000 086(5) cm−1, B10=0.026 786(4) cm−1, and ξ10=−0.049(3) for the 1010 transition based on the literature value of A0.

Multiwavelength Monitoring of Photofragment Fluorescence after 193 nm Photolysis of NaCl and NaOH: Application to Measuring the Sodium Species Released from Coal at High Temperatures

Excimer laser photodissociation of gas-phase NaCl and NaOH, and monitoring of the subsequent Na photofragment fluorescence, are used to determine the concentration of the species in coal-derived gaseous environments. Detection limits lower than 1 ppb of NaCl have been achieved under atmospheric conditions using 193 nm photodissociation. It is shown that monitoring two Na emission wavelengths (at 819 and 589 nm) allows speciation between NaOH and NaCl in the gas phase. In particular, emission from the Na 3[sup 2]D levels (at 819 nm) has been unambiguously attributed to photodissociation of NaOH. This emission results from hot-band absorption of the excimer laser, and thus its intensity is temperature dependent and weaker than 3[sup 2]P (589 nm) emission. The technique has been applied to the detection of NaCl and NaOH released during the pyrolysis and gasification of samples of Loy Yang (Australian brown) coal. 13 refs., 9 figs., 1 tab.

Sub-Doppler Heterodyne Frequency Measurements of Carbonyl Sulfide Transitions between 56 and 62 THz

We report the first sub-Doppler frequency measurements on carbonyl sulfide (OCS) between 56 and 62 THz (1886 and 2064 cm −1). The experiment utilizes heterodyne techniques to measure the frequency of a CO laser, which is locked to saturation dips of OCS transitions at low pressure, against combination frequencies of two saturation-stabilized CO 2 lasers. For these measurements, ground state and hot band absorptions, we achieved absolute accuracies between 17 and 48 kHz. We fitted the results together with the existing data set to determine improved OCS transition frequencies for calibration of spectra in the 2000-cm −1 region. In addition, by combining our new hot band measurements with existing frequency data for the lower energy levels, we provide improved transition frequencies for higher wavenumber regions. New calibration tables are given at 1890, 2050, and 2730 cm −1.

Hot bands in overtone absorption of pyrrole, methyl isocyanide and isobutane: Photoacoustic spectra at 140 C

The intracavity laser photoacoustic spectra of the 4νN–H and 4νC–H overtones of pyrrole, the 5νC–H of methyl isocyanide and acetonitrile, the 5νC–H and 6νC–H of isobutane, and the 6νC–H of cyclohexane were measured at temperatures up to 140 C (T140) and compared to the room temperature (Trt) spectra. Hot bands were identified by their temperature dependence in pyrrole, methyl isocyanide, and isobutane. From the temperature dependence of the relative intensities of the hot band compared to the main band, the following average wave numbers of the thermally populated states were deduced: 340, 300, and 208 cm−1 for pyrrole, isobutane, and methyl isocyanide, respectively. These values compare well to the bending motion frequencies in these molecules. Large redshifts of the hot bands from the main peak were observed, as expected for high overtone transitions due to large anharmonicities. The anharmonic constants determined from these shifts are −25 cm−1 for the N–H in pyrrole and −20 cm−1 for the C–H in methyl isocyanide and isobutane. In the case of the N–H absorption the hot band was completely separated from the main transition. In the C–H overtone spectra these hot band absorptions were not separated and result in an apparent increased width of the overtone transition.

Vibrational lifetimes of surface hydroxyl groups of zeolites by picosecond IR pulses

Vibrational lifetimes of the surface hydroxyl groups of zeolites (Y-type and mordenite) were measured in real time by pump-probe absorption spectroscopy using picosecond IR pulses. The relaxation lifetimes of the OH (ν=1) groups of Y-type zeolites which give the OH stretching bands at 3625 and 3540 cm −1 were 180 ± 12 and 63 ± 14 ps, respectively. The influence of the micro-structure of zeolites on the lifetime is discussed. For the surface OH groups of mordenite, the decay time of the transient hot band absorption at 3457 cm −1 (ν=2←1) was also measured by pumping the fundamental band at 3608 cm −1 to be 154 ± 18 ps, which agreed with 161 ± 18 ps obtained from the bleaching experiment of the fundamental band.

Spectra and energy levels ofTm3+:Y3Al5O12

Absorption spectra of ${\mathrm{Tm}}^{3+}$:${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ are reported between 1.9 and 0.26 \ensuremath{\mu}m at 15 and 90 K, and between 0.80 and 0.35 \ensuremath{\mu}m at 1.6 K. Laser-excited emission obtained at 80 K is also reported from the ${\mathrm{Tm}}^{3+}$ manifolds $^{1}\mathrm{D}_{2}$, $^{1}\mathrm{G}_{4}$, $^{3}\mathrm{H}_{4}$, and $^{3}\mathrm{F}_{4}$ to the ground-state manifold, $^{3}\mathrm{H}_{6}$. The emission from $^{1}\mathrm{D}_{2}$ also includes transitions to Stark levels in manifolds $^{3}\mathrm{F}_{4}$, $^{3}\mathrm{F}_{3}$, and $^{3}\mathrm{F}_{2}$. Analysis of the emission spectra confirms the experimental crystal-field splitting deduced from an analysis of the hot-band absorption data. Both emission and absorption spectra indicate that ${\mathrm{Tm}}^{3+}$ ions occupy several different sites although the majority of ${\mathrm{Tm}}^{3+}$ ions appear to substitute for ${\mathrm{Y}}^{3+}$ ions in dodecahedral lattice sites (${D}_{2}$ point-group symmetry). The most intense spectra are analyzed assuming selection rules for ${D}_{2}$ symmetry. A lattice-sum calculation predicts a symmetry of ${\ensuremath{\Gamma}}_{2}$ for the ground state. Using this result the symmetries of 20 ${\ensuremath{\Gamma}}_{1}$, 11 ${\ensuremath{\Gamma}}_{2}$, 17 ${\ensuremath{\Gamma}}_{3}$, and 18 ${\ensuremath{\Gamma}}_{4}$ Stark levels were identified experimentally and compared with results from a crystal-field splitting calculation. A Hamiltonian consisting of Coulombic, spin-orbit, interconfiguration-interaction, and crystal-field (${D}_{2}$ symmetry) terms was parametrized and diagonalized for all manifolds of the ${\mathrm{Tm}}^{3+}$(4${\mathrm{f}}^{12}$) configuration. The rms deviation between 66 experimental and calculated Stark levels was 11 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Infrared Multiple Photon Absorption of Silicon Tetra‐Fluoride and Anisole

AbstractThe infrared multiple absorption probabilities of the title molecules are measured at two frequency ranges: one for which the single photon absorption (SPA) cross section is large (range I), and another for which it is much smaller (range II). For SiF4, the high power absorption probability in range I is much smaller than the SPA cross section. Collisions significantly increase the average number of photons absorbed. In range II, the absorption cross section increases upon increasing the laser fluence. For anisole, the same fluence effect is qualitatively observed, but it is less pronounced in range I. For anisole pressure exceeding 0.1 torr collisions with argon do not change the average number of photons absorbed per molecule. The 10.2 μ absorption in SiF4 is suggested to be initiated by a single photon hot band absorption, followed by absorption in the quasi‐continuum (QC). QC absorption cross sections are in the range of (5–20) × 10−21 cm2 for both SiF4 and anisole.

Investigation of the absorptions of monoisotopic OsO4 with CO2 and N2O laser lines with saturation spectroscopy

Some absorption features of monoisotopic 186–190,192OsO4 have been studied by saturated absorption using low-pressure CO2 and N2O lasers. Ground-state absorptions of the ν3 fundamental band were distinguished from hot-band absorptions by investigation of the linear absorption coefficients, and were assigned on the basis of frequency calculations. In addition the vibrational transition moment was evaluated. A few lines were found, which, due to their unusual saturation properties, were identified as forbidden transitions arising from off-diagonal terms of a sixth rank tensor in the effective Hamiltonian. Their observation permits a value of the tensor centrifugal distortion constant to be derived: Dt = (2.045 ± 0.018) × 10-9 cm-1.

Vibrational energy redistribution in glyoxal following internal conversion

The vibrational redistribution of energy following internal conversion in glyoxal has been studied in a molecular beam using the ’’pump-and-probe’’ technique. A beam of glyoxal is initially pumped from So to a selected vibronic level of S1. After internal conversion occurs, a tunable dye laser is used to probe the hot band absorption of the molecules by re-excitation into S1. The total fluorescence as a function of probe laser wavelength is observed. The excitation spectrum is broad and featureless in the red but shows sharp structure in the vicinity of the pump laser wavelength. However, the sharp structure depends on the portion of the rotational contour excited. The experimental results show that the redistribution of energy is nonstatistical among the isoenergetic levels. This is explained by assuming that combination bands of low quanta of vibrations are the accepting levels in the internal conversion process and that these accepting levels undergo slow energy redistribution.

CF4laser oscillator-amplifier measurements: Small signal gain and self absorption

Some of the fundamental parameters associated with the CF(4) optically pumped laser were measured with the aid of an oscillator-amplifier arrangement. The small signal gain for the strongest P-branch line was determined for pure CF(4) and for CF(4) in the presence of He and CO buffer gases at different pressures. The self-absorption of the laser radiation by CF(4) was determined for various P-, Q-, and R-branch lines at several temperatures. The data were used to determine that the loss mechanism for the laser process is associated with hot-band absorption from the 1 nu(2) level.

CF_4 laser oscillator–amplifier measurements: small signal gain and self-absorption

Some of the fundamental parameters associated with the CF(4) optically pumped laser were measured with the aid of an oscillator-amplifier arrangement. The small signal gain for the strongest P-branch line was determined for pure CF(4) and for CF(4) in the presence of He and CO buffer gases at different pressures. The self-absorption of the laser radiation by CF(4) was determined for various P-, Q-, and R-branch lines at several temperatures. The data were used to determine that the loss mechanism for the laser process is associated with hot-band absorption from the 1 nu(2) level.

HCO production, vibrational relaxation, chemical kinetics, and spectroscopy following laser photolysis of formaldehyde

Formaldehyde vapor was photolyzed with a tunable pulsed uv laser. Flash kinetic absorption spectra of the HCO produced were recorded by intracavity dye laser spectroscopy with a time resolution of 1 μs. The energy threshold for radical production was confirmed to be at 86±1 kcal/mole. Photolysis at 294.1 nm produced HCO in its ground vibronic state (∼2/3) and with one quantum of vibrational excitation in either the bending (∼1/3) or CO stretching (10−1–10−2) vibrations. Observation of the CO stretching hot band absorptions allowed that frequency to be determined as 1868.4±1 cm−1. Quantitative, state-resolved measurements of concentration vs time were made in pure H2CO and in mixtures with O2, NO, or Ar. The vibrational relaxation rate for the bending vibration of HCO in collisions with H2CO was (4.3±1) ×10−12 cm3 molecule−1 s−1. Reaction rates for HCO+NO→HNO+CO and HCO+O2→HO2+CO were measured as (1.4±0.2) ×10−11 and (4.0±0.8) ×10−12 cm3 molecule−1 s−1, respectively. Approximate rates were determined for the radical–radical reactions H+HCO→H2+CO and 2HCO→H2CO+CO as 10−9.26±0.3 and 10−10.2±0.5, respectively.

Optical Q-branch pumping of the P- and R-branch transitions in the ν 3 mode of SF 6

One consequence of pumping the Q-branch of the ν 3 mode of SF 6 heretofore not examined is the production of substantial gain on certain low J R-branch transitions. In the analysis, the small-signal gain equation is derived for an individual symmetry species within a rotational manifold, since the nuclear spin statistical weigfhts play a significant role in the gain. The large value for the gain is a consequence of the selection rules governing the symmetry species and the way the spin statistical weights vary with the rotational quantum number J. It is shown that the transitions with gain, although only a fraction of a wave number removed from the pumped Q-branch, should be observable under appropriate conditions. For example, at a temperature of 135 K and a pump intensity of 10 kW/cm 2, the transitions with gain are not obscured by either hot-band absorption or by power broadening of the Q-branch. This induced gain may be of importance for the multiple photon dissociation of SF 6.

Infrared multiphoton dissociation of DN 3 and HN 3: Formation and reaction of electronically excited ND

Chemiluminescence is observed from DN 3 at pressures below 100 mtorr following irradiation with the focused output of a CO 2 TEA laser. Emission is attributed to ND 2( 2A I) formed in the reaction ND(a 1Δ) + DN 3 → ND 2 ( 2A 1) + N 3. The ND(a 1Δ) is produced in the primary photolysis. Time resolved studies of the fluorescence permit determination of the rate constant for the chemiluminescent reaction (2.09 ± 0.31 μs −1 torr −1). Multiphoton dissociation of HN 3 by use of a laser wavelength coincident with a hot band absorption is also demonstrated.

Absorption of the 3.39 μm He-Ne laser line by methane from 300 to 2400 K

The absorptivity of methane for the 3.39 μm He-Ne laser line was measured over the temperature range 300–2400 K using shock tube techniques. Theoretical understanding of the temperature, pressure and composition dependence could be achieved up to 1500 K with reasonable values of the line-shape and line-shift parameters. Above 1700 K, hot-band absorption from an unidentified excited state at ∼7000 cm -1 apparently begins to dominate the absorption.

Infrared saturable absorbers. Part II: Intensity dependent transmission of SF6 at 936.8 wave numbers

Due to rotational hole burning Q-switched laser pulses sometimes saturate a molecular absorber more effectively than CW laser radiation. The absorption by SF6 of frequencies near the P(28) line in the 10.6 μm CO2 laser band provides an interesting example. In this case it has been reported elsewhere that for peak intensities [Formula: see text] an increase in pulse intensity produces a monotonic increase in transmission whereas the CW transmission decreases monotonically with intensity. We report new measurements made over a wider range of laser power levels and at several SF6 pressures, which show that the global behavior of the transmission as a function of average beam power is quite similar for both CW and pulsed radiation. The pulse transmission is somewhat greater at all power levels, but when the average power is sufficiently high the growth of hot-band absorption and possibly other intensity dependent changes in the ground and excited-state absorption cross sections cause the transmission to fall well below its low-intensity value for CW radiation and pulses alike. The experimental transmission curves for both cases display as many as three extrema and cannot be explained, even qualitatively, by the five-level model used in the earlier work. The extended results are consistent with the modified five-level model presented in Part I of this pair of papers.

ChemInform Abstract: HOT BAND TRANSITIONS TO THE TRIPLET STATE IN ANTHRACENE AND PYRENE CRYSTALS

The low energy end of the S0→T1 absorption spectrum has been measured in anthracene and pyrene crystals from 220 to 330°K. A transition is found in both of these systems at an energy approximately 400 cm−1 below the 0–0 energy. The temperature dependence of this absorption reveals it to be excitation of a hot vibrational level of the ground state. Polarized excitation measurements have been carried out on the hot band absorption in anthracene, and it is found that the polarization ratio between light polarized parallel and perpendicular to the b axis is the same as that for the 0–0 transition. In addition it is found that there is no measurable shift (within 4 cm−1) in the position of the hot band maxima for the two polarizations. Within the framework of strong and weak coupling and the Born‐Oppenheimer approximation only weak coupling is found to fit all the experimental facts.

Hot band transitions to the triplet state in anthracene and pyrene crystals

The low energy end of the S0→T1 absorption spectrum has been measured in anthracene and pyrene crystals from 220 to 330°K. A transition is found in both of these systems at an energy approximately 400 cm−1 below the 0–0 energy. The temperature dependence of this absorption reveals it to be excitation of a hot vibrational level of the ground state. Polarized excitation measurements have been carried out on the hot band absorption in anthracene, and it is found that the polarization ratio between light polarized parallel and perpendicular to the b axis is the same as that for the 0–0 transition. In addition it is found that there is no measurable shift (within 4 cm−1) in the position of the hot band maxima for the two polarizations. Within the framework of strong and weak coupling and the Born-Oppenheimer approximation only weak coupling is found to fit all the experimental facts.

Comment on the applicability of Urbach's rule to molecular crystals

The extent to which the absorption profile below the first exciton band can be predicted without recourse to resonance transfer and strong exciton—phonon coupling is evaluated for a crystal transition derived from a moderately intense free molecule transition. It is argued, on the basis of a simple model, that Urbach rule behavior can arise from the same class of weak exciton—phonon interactions that determine hot band absorption shapes.