Welf A. Kreiner

Infrared spectroscopy of carbo‐ions. III. ν3 band of methyl cation CH+3

The infrared spectrum of the degenerate ν3 vibration–rotation band of methyl cation CH+3, one of the most fundamental carbo‐ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm−1 using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H2:CH4∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air‐cooled and a water‐cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH+3 appeared amid many other lines of C2H+2 and C2H+3 but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH+3 and the isotopic species 13CH+3. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as exp...

Sub-doppler spectroscopy of the ν2 band of NH3 using microwave modulation sidebands of CO2 laser lines

Microwave modulation sidebands of CO2 laser lines were used for sub-Doppler saturation spectroscopy in the ν2 fundamental of NH3. Fifteen transitions were observed and their positions were determined with an absolute frequency accuracy of typically ±100 kHz. The intensities of the observed Lamb dips and crossover dips could be shown to be consistent with theory. In addition, the electronic quadrupole hyperfine splittings due to the 14N nucleus on five transitions were resolved. From this the quadrupole coupling constant eqQ in the ν2 state was determined as (eqQ)(v2=1,s) = −4442 ± 156 kHz and (eqQ)(v2=1,a) = −4329 ± 375 kHz for the lower and upper inversion levels, respectively. The observed increase of 296 kHz in the magnitude of eqQ upon vibrational excitation could be shown to be consistent with its dependence on hybridization and ionicity of the NH bond.

Observation of the infrared spectrum of methyl cation CH+3

The vibration‐rotation infrared spectrum of the ν3 fundamental band of the methyl cation CH+3 has been observed using a difference frequency laser system. The analysis of the spectrum demonstrates the D3h symmetry of the ion. This work marks the beginning of high resolution spectroscopy of simple polyatomic hydrocarbon cations.

Inverse Lamb dip spectroscopy using microwave modulation sidebands of CO2 laser lines

We report the observation of infrared Lamb dips using a widely tunable laser source. The microwave sidebands generated on CO2 laser lines provide sufficient power (∼0.5 mW) and spectral purity (Δν≲100 kHz) for saturation spectroscopy. Application of this infrared source to the observation of infrared Lamb dip spectra of CH3F and SiF4 is reported.

Observation of second‐order Stark effects in silane, SiH4

The Stark effect on rovibrational transitions of the tetrahedral molecule silane has been studied by laser Stark spectroscopy, i.e., by applying dc fields of up to 65 kV/cm and monitoring the absorption of fixed frequency ir laser lines. The coincidences observed occur between the ν4 fundamental of SiH4 and N2O laser lines. Using the inverse Lamb dip technique single M components of three high J transitions could be resolved, and two of these have been identified by rf–ir double resonance. In both cases the Stark shift is due to the second‐order Stark mixing of A1 with A2 rotational levels. The analysis of the Stark effects is consistent with the ground state centrifugal distortion moment ϑzxy=3.34×10−5 D measured previously by Kagann, Ozier, and Gerry. Experiments to measure the frequency offset Δν=νM−νL of the molecular absorption lines from the nearest laser line have also been performed, using the beat between two N2O lasers. Direct evidence of the second‐order Stark shift has been obtained by observi...

Vibration‐induced first‐order Stark effect in tetrahedral molecules: SiH4 and SiF4

The vibration‐induced permanent dipole moments in SiH4 and in SiF4 have been determined by measuring the first‐order Stark effect on the infrared vibration–rotation lines in the triply degenerate ν4 and ν3 fundamental bands, respectively. Microwave modulation sidebands on CO2 laser lines were generated with sufficient power and spectral purity to be used as the frequency‐tunable infrared source for the sub‐Doppler saturation Lamb dip measurements. Theory is also presented for calculating the dipole moment matrix elements in the excited vibrational state. From the observed Stark splittings, the vibration‐induced permanent dipole moment P has been determined to be 1.46(5)×10−2 D for SiH4 in the v4=1 state, and 4.21(12)×10−2 D for SiF4 in the v3=1 state. In addition, Watson’s coefficient for the rotation‐induced permanent dipole moment θxyz has been determined to be 2.97(19)×10−5 D in SiH4 (v4=1).

Observation of the isotope effect in the ν2 fundamental of germane

Ten ro–vibrational transitions in the ν2 fundamental of germane, GeH4, have been investigated with a tunable sideband laser. The isotope effect, i.e., the fivefold occurence of each transition due to the natural germanium isotopes, has been clearly resolved. From the observation of a combination difference the ground state centrifugal distortion constant Dt is determined to 67.6±0.4 kHz for two molecular isotopes separately. The dependence of other molecular constants on the isotope mass is shown. The worst‐case error in absolute frequency measurement is about 18 MHz while the relative accuracy is believed to be better than 5 MHz for most of the lines observed.

Laser sideband spectroscopy in the ν4 fundamental of silane, SiH4

Abstract We performed direct frequency reading IR spectroscopy in the ν 4 [R_(14), A2(1) ← A1(1)] transition-frequency region of SiH 4 around 945.950 cm −1 by electrooptic mixing the stabilized CO 2 -laser line P(20) with a tunable millimeter-wave signal. Power of as much as 100 μW was available in the tunable sideband allowing direct detection of the IR signal. Two strong absorption lines were observed at (945.972 ± 0.001) cm −1 and at (945.995 ± 0.001) cm −1 ; their pressure broadening rates were found out to differ from each other by a factor of about 2. Accuracy in absolute IR-frequency assessment of better than 10 −3 cm −1 was achieved simultaneously providing providing 10 −4 cm −1 resolution.

IR laser sideband observations in GeH4 and CD4

We have used the technique of IR laser sideband spectroscopy to observe single ro–vibrational transitions in the ν2 fundamental of GeH4. The isotope structure has been resolved within the Doppler width of the lines. The isotopic frequency shift for the J″=7 transition in the R branch of ν2 was found to be 120 MHz/amu. This value is much smaller than the isotope shift in the ν4 fundamental (0.22 cm−1 =6.5 GHz/amu) which has been observed with a conventional high resolution grating spectrometer. For GeH4 a line width (FWHM) of 44.4 MHz was found which compares well with the Doppler width (41 MHz) at 300 K. The application of this spectroscopic method was extended to some transitions in the ν4 fundamental of CD4. In total 32 transitions have been observed in GeH4 and 17 in CD4. The absolute accuracy in frequency measurement is ? ±60 MHz.

29SiH4 and 30SiH4: Dipole moment parameters of the dyad from Stark effect observations with laser sidebands

Abstract The linear Stark effect within the ν 2 ν 4 dyad of 29SiH4 and 30SiH4 has been investigated by applying the infrared sideband technique at microwave modulation frequencies. Two vibration-induced dipole moment parameters have been measured for each isotopomer on natural abundance samples.