A simultaneous analysis of the microwave,submillimeterwave, and infrared transitions between the ground and ν2 inversion-rotation levels of 15NH3

The submillimeterwave spectra of the pure inversion and inversion-rotation transitions in the ν₂ excited state (79 transitons) and the diode laser spectra of the ν₂ band (83 transitions) of ¹⁵NH₃ have been measured. A simultaneous least squares analysis has been carried out of these data together with previously published wavenumbers of the pure inversion transitions and inversion-rotation transitions in the ground state measured by the microwave and Fourier spectroscopy, and the ν₂ band transition frequencies obtained by the infrared-microwave two-photon technique. A theory of the Δk = ±3n interactions in the ground and ν₂ excited states of ammonia (?. Urban, V. ?pirko, D. Papou?ek, J. Kauppinen, S. P. Belov, L. I. Gershtein, and A. F. Krupnov, J. Mol. Spectrosc.88, 274–282 (1981)) has been used in the analysis. The “smoothed” values of the ν₂ band wavenumbers can be used for calibration purposes with better than 1 × 10^?3 cm^?1 precision.     

Δk = ±3 “forbidden” infrared transitions in the ν2-band of NH3

Two Δk = ±3 “forbidden” vibration-rotation transitions in the ν₂-band of NH₃ have been measured by using infrared-microwave two-photon spectroscopy and laser Stark spectroscopy. Combining these results with Rao's recent measurement of the band, we have obtained the C₀ rotational constant of 6.2280 ± 0.0008 cm^?1.     

Doppler-free two-photon spectroscopy of the 2ν3 band of SF6

The assignments of most of the observed Doppler-free two-photon spectra of the 2ν₃ band of SF₆ near the P(14)–P(20) lines of a 10-μm CO₂ waveguide laser are reported. The rovibrational and anharmonic constants for this band were obtained, along with ground state constants determined from observed “forbidden” transitions.     

High-resolution infrared and microwave spectroscopy of the ν4 and 2ν2 bands of 14NH3 and 15NH3

More than two thousand Stark resonances of the ν₄ and 2ν₂ band transitions of ¹⁴NH₃ and ¹⁵NH₃ were observed at Doppler-limited resolution with a CO laser. Fourier transform infrared spectroscopy on ¹⁵NH₃ is also carried out. Thirty-six new microwave transitions including seven perturbation-enhanced transitions are observed in the v₄ = 1 excited vibrational state of ¹⁴NH₃ and ¹⁵NH₃. Accuracies of all available spectroscopic data on the v₄ = 1 and the v₂ = 2 states are evaluated and analyses of the vibration-rotation spectra are performed. The Coriolis interaction between the closely lying v₄ = 1 a (antisymmetric level) and v₂ = 2 s (symmetric level) states is explicitly included in the analysis. Smaller Coriolis interactions between the v₄ = 1 a and the v₂ = 1 s states and between the v₂ = 2 s and the v₂ = v₄ = 1 a states (i.e., (v₁, v₂, v′₃, v′₄) = (0 1 0⁰ 1¹)) are also taken into consideration. The accuracy in determination of the principal molecular constants is 10^?6. The parameters thus obtained reproduce the frequencies of the vibration-rotation transitions and inversion transitions within the accuracy of 0.0024 cm^?1.     

Laser Stark saturation spectroscopy of the ν2 band of CD3l

The ^QP(4) transitions of the ν₂ band of ¹²CD₃I, which lie close to the P(16) 10.4-μm CO₂ laser line, have been observed by saturation spectroscopy using both an intracavity Stark cell, and a Shimizu-type multipass arrangement. Since both level crossing and Lamb dip signals can be seen using these transitions, they serve to illustrate both the uses of the two types of Stark cell, and also the differences between the two kinds of nonlinear signals. A simple derivation of the quadrupole matrix elements in the uncoupled basis is given. The dipole moments of the molecule in the ground and ν₂ states were determined to be 1.656₂ and 1.652₃ D, from the measurements of the very sharp intracavity level crossing signals.     

The 923-cm−1 band of CF235Cl2 (freon 12), studied by infrared-microwave double resonance

The constants of the 923-cm^?1 band (ν₆) of CF₂³⁵Cl₂ were accurately determined by the application of infrared-microwave double resonance using CO₂ and N₂O lasers. The frequencies of 32 ground-state and 34 (v₆ = 1) state rotational transitions and 14 infrared transitions were measured. The infrared transition frequencies were generally obtained with an accuracy of ± 20 MHz, but in some cases observation of two-photon Lamb-dips allowed the accuracy to be improved to ± 5 MHz. Many double-resonance signals displayed a predominantly “dispersion-type” lineshape and it has been shown that the phase of the observed signals gives information over the relative disposition of the energy levels involved.     

Optoacoustic laser Stark spectroscopy in the ν2 band of 14NH3

Laser Stark spectroscopy of the R(5, K) transitions in the ν₂ band of ammonia was carried out using coincidences with the 9-μm band CO₂ laser lines. We observed 22 Doppler-free resonances by using an optoacoustic detector and a Lamb-dip stabilized CO₂ laser. A simultaneous analysis of sa, aa, and ss lines yields zero-field transition frequencies with an absolute accuracy of 1 ～ 2 MHz.     

Inversion and inversion-rotation spectrum of 14NH3 in the ν2 excited state

Using the vibration-inversion-rotation Hamiltonian for ammonia [V. ?pirko, J. M. R. Stone, and D. Papou?ek, J. Mol. Spectrosc.60, 159–178 (1976)], a modified theory is worked out for the Δk = ±3n interactions between the inversion-rotation energy levels of NH₃ which takes into account the large amplitude inversion motion. Eighty frequencies of inversion and inversion-rotation transitions and two perturbation-allowed transitions in the ν₂ state of ¹⁴NH₃ are measured in the far-infrared region around 1 THz (≈33 cm^?1), mostly with the microwave accuracy, by the radiofrequency spectrometer with acoustic detector. By a least-squares fit of these data and the data of the infrared-microwave two-photon and infrared heterodyne measurements of the ν₂ band, a set of molecular constants for the ν₂ state of ¹⁴NH₃ is obtained which reproduces the submillimeterwave data with the accuracy of the experiment.     

Microwave-optical double resonance in NO2

A series of experiments performed to establish the nature of the previously reported microwave optical double resonance signals in electronically excited nitrogen dioxide are reported. The DC stark effect, hyperfine spectrum, and microwave saturation behavior of the double resonance signals observed by exciting NO₂ with the 4880 Å line of an argon ion laser have been studied. The data presented support the assignment of this double resonance signal as occurring within the rovibronic manifold of the ²B₂ electronic state. The observation of a number of new excited state microwave transitions is also reported.     

Laser-microwave double-resonance spectroscopy in CF3I with four CO2-laser lines

Double-resonance effects produced by three CO₂-laser lines, the 9.4-μmR(12),R(14), and R(18), have been newly observed. From an analysis of the observed laser-induced changes in intensity of various microwave transitions, the infrared transitions which were accidentally coincident with these three laser lines were determined. In every case an exact rotational assignment of the infrared transition was possible, but in some cases the assignment of the vibrational band involved was difficult. When the previously discussed (l) effects produced by the R(16) laser line are included these four consecutive laser lines pump a total of seven infrared transitions of CF₃I. It appears that all seven transitions belong to different fundamental, combination, and hot bands.     

The ν2 band of 14NH3: A calibration standard with better than 1 × 10−4 cm−1 precision

The Fourier transform spectra and the diode laser spectra of the ν₂ band of ¹⁴NH₃ have been measured with 0.005 and 0.002 cm^?1 resolution, respectively. A simultaneous least squares analysis has been carried out of these data together with the microwave, submillimeterwave, diode-laser heterodyne, and infrared-microwave two-photon transition frequencies between the ground and the ν₂ inversion-rotation levels. A theory of the Δk = ±3n interactions in the ground and ν₂ excited states of ammonia (?. Urban, V. ?pirko, D. Papou?ek, J. Kauppinen, S.P. Belov, L. I. Gershtein, and A. F. Krupnov, J. Mol. Spectrosc.84, 288–304 (1981)) has been used in the analysis. The “smoothed” values of the ν₂ band wavenumbers can be used for calibration purposes with better than 1 × 10^?4 cm^?1 precision. On the basis of these results, a critical evaluation has been carried out of several experimental techniques of very high resolution infrared spectroscopy.     

Vibrational heating in the v2 mode of ammonia by a CO2 laser

Various levels of the v₂ mode of ammonia have been pumped by a pulsed CO₂ laser. In mixtures of NH₃ with a large excess of Ar the 1 v₂ (+), 1 v₂ (?), and 2 v₂ (?) levels could be appreciably populated. The level populations were monitored by absorption spectroscopy in the ultraviolet using appropriate vibronic transitions of the ùA^″₂←X?¹A₁ absorption band . The energy balance was examined by comparing the number of molecules removed from the ground state with the number densities in the vibrationally excited levels and with the number of photons absorbed during the laser pulse. While the ground-state depletion corresponds well to the excited-state populations only a fraction of the photons deposited can be accounted for in the excited molecules. This is consistent with the assumption of very fast V-T relaxation.     

Infrared laser magnetic resonance (LMR) spectroscopy of NO2 (ν2) with a CO2 laser

The infrared laser magnetic resonance spectra for the ν₂ band of NO₂ were observed by using a CO₂ laser. High-K vibration-rotation transitions from ^rR₆(N) to ^rR₁₁(N) (v₂ = 1 ← 0) were observed. The analysis yielded some molecular parameters including two g factors for the excited vibrational state (v₂ = 1).     

Radiofrequency spectroscopy of AsH3 inside a CO2N2O laser cavity

The method of radiofrequency spectroscopy inside a CO₂N₂O laser cavity has been applied to the observation of pure nuclear quadrupole resonance, A₁-A₂ transitions and two-photon transitions of AsH₃. From the assignment of these transitions some coincidences between laser lines and Δk = 3 transitions in the ν₂ band and Δ|k ? l|; = 3 transitions in the ν₄ band have been assigned. The quadrupole coupling constant eqQ and its rotational dependence χ_J, χ_K and χ_d, and the spin-rotation constants C_N and C_K for the As nucleus have been determined for the ground state. The effective values of eqQ has been determined for the ν₂ and the ν₄ states.     

Infrared-microwave two-photon spectroscopy of the ν2 band of 14NH3

Transitions in the ν₂ band of ¹⁴NH₃ were recorded by means of an infrared laser microwave two-photon spectrometer. The spectrometer, which uses a minicomputer to step the microwave frequency and record the spectrum, is described. With sample cells outside the laser cavity good lineshapes are obtained, so that the accuracy of frequency measurement was limited by the resettability of the CO₂ or N₂O lasers employed, ～0.0002 cm^?1. The present data are compared to previously obtained results based on CO₂ or N₂O laser frequencies and to recently reported calculations. Rotational constants derived for the hypothetical inversion-free ground and v₂ = 1 states are reported.     

Microwave spectroscopy of propynal in the v2 = 1 vibrational state by IR-MW triple resonance

Infrared-microwave triple resonance was applied to microwave spectroscopy of propynal, HCCCHO, in the v₂ = 1 vibrational state. Eleven microwave transitions were newly observed by using a Zeeman-tuned 3.51 μm HeXe laser as the infrared radiation source. The spectrum was analyzed including the 11 transitions previously observed by double resonance. A least-squares fit of the 13 low-J transitions revealed that the rotational constants in the K_?1 = 1 states were slightly different from those of the K_?1 = 0 states. A higher-order rotational resonance was proposed for explanation of the effect.     

Laser-microwave two-photon and double resonance spectroscopy of two linear molecules; HCCF and FCN

The technique of laser-microwave two-photon spectroscopy has been used to determine the frequencies of a number of transitions in fundamental and hot-bands of the two linear molecules HCCF and FCN. This method has previously only been applied in a systematic way to spectroscopy in NH₃ and it was the aim of this investigation to determine to what extent this method was applicable to more normal molecular species. The maximum effective tuning range produced with these molecules was ±3.7 GHz with a microwave power density of ?100 mW/cm² and a laser power density of ?40 W/cm². Transition frequencies were determined with accuracies up to ±3 MHz using a Lamb-dip technique. In the case of fluoroacetylene the observation of a number of double resonance signals involving direct l-type doubling transitions allowed determination of l-type doubling constants for the states (v₃ = 1, v₄ = 1) and (v₃ = 1, v₅ = 1). The P(12) C¹⁸O₂ laser line was shown to lie within the Doppler width of the upper l-type doublet R(25) transition of the (ν₃ + ν₄) ? ν₃ hot band. In this particular case velocity-tuned multiple photon dips were observed. The dispersive component of double resonance and two-photon Lamb-dip signals was observed to have opposite phase in two different vibrational states. The possible diagnostic value of this observation is discussed.     

Infrared-microwave two-photon spectra of the ν3 bands of 12CH3F and 13CH3F

Infrared-microwave two-photon spectra have been obtained for the ν₃ bands of ¹²CH₃F and ¹³CH₃F with a two-photon spectrometer employing a CO₂ laser and a computer-coupled microwave source operating in the 8–18 GHz region. Even though the intensities of the spectra for the double parity levels in these molecules are inversely proportional to the square of the microwave frequency, transitions have been observed with microwave frequencies of up to 16 GHz. Comparison of these observed two-photon frequencies to frequencies predicted from infrared laser Stark spectroscopy, and to frequencies calculated from vibration-rotation parameters obtained by fitting these and other frequencies, shows agreement to within a few MHz. Spectroscopic parameters for the ground and ν₃ excited states of the two species are reported.     

Identification of forbidden vibration-rotation transitions in 15NH3

Forbidden Δ|k ? l| = 3 vibration-rotation transitions have been observed in the ν₄ band of ¹⁵NH₃. The analysis of these transitions, together with previously published data on the allowed transitions, has made it possible to determine a set of molecular parameters, including for the first time the rotational constant C as well as the centrifugal distortion constants D_K and H_KKK, which are necessary for the calculation of energy levels. Some weak forbidden transitions in the ν₂ band have also been observed.     

Collisional transfer of rotational energy in the 2B1 electronic state of nitrogen dioxide

Collisional satellite lines have been observed in fluorescence from nitrogen dioxide excited by the 4545-Å line of the argon laser. The 13_0,13 level of the (0, 8, 0) vibrational state is populated by the laser and undergoes collisionally induced transitions to the 11_0,11, 15_0,15, and 17_0,17 states. These collisionally populated states are identified by their fluorescence to the well-studied (0, 0, 0) and (0, 1, 0) levels of the ground electronic state. These satellite lines are also observed in fluorescence to the (0, 2, 0) and (0, 3, 0) vibrational levels of the ground electronic state. The wavenumbers of those lines, together with those from unrelaxed fluorescence and previously published microwave transitions, allow vibrational and rotational constants for the higher vibrational states to be determined more accurately than was previously possible. Several much weaker forbidden transitions have also been observed, including ΔK_a = 0 through ?6 transitions in the (0, 8, 0)-(0, 0, 1) band.