Lasers vibrational-rotational transitions

Unless the cavity is tuned to a particular wavelength the vibration-rotation transition with the highest gain is the P-branch transition involving the rotational level which has the highest population in the 3 state. This is P(22), with J" = 22 and J = 21, at normal laser temperatures. The reason why this P-branch line is so dominant is that thermal redistribution of rotational level populations is faster than the population depletion due to emission. 

The earliest experiments with lasers in absorption spectroscopy were performed with the high-gain infrared line X = 3.39p of the He-Ne laser the first gas laser Several authors Miscovered that this laser line is absorbed by many hydrocarbon molecules, causing a vibrational-rotational transition in a band which belongs to the excitation of a C-H stretching vibration . ... 

In some cases the measured V-V transfer rates differ from theoretical predictions, which indicates that some improvement is needed in theoretical models. Sophisticated computer analysis has to be employed to transform the experimental data to theoretical parameters The spectroscopic studies of cw and pulsed chemical lasers including the local variation of laser output on different vibration-rotation transitions as a function of distance froih the injectory array has been a useful tool, too, for elucidating the different reaction paths and the excited molecular levels involved... 

Population inversions have been observed in a number of chemical and photochemical reactions. In a few of these cases, laser action has been produced in a suitable cavity. In most cases of molecular laser emission, there is only partial inversion282 in which several vibration-rotation transitions are inverted even though the total population in the upper vibrational state does not exceed that in the lower. In this case there is laser action in P branch transitions only. 

Most known chemical lasers oscillate on vibration-rotation transitions of a hydrogen halide. The first such laser was driven by the flash initiated explosion of H2 + C12 mixtures287. Here the flash dissociates the Cl2 to start the chain decomposition, and the population inversion is due to the subsequent reactions... 

Laser action is observed on vibration-rotation transitions of CO in the flash photolysis of CS2 + 02 mixtures61. Ay = 1 transitions are observed with y in the range 6-14. Only P branch lines are observed, as usual. The excitation is presumably chemical rather than by energy transfer to ground state CO. A suggested mechanism involves... 

Figure 11.46. (a) Recording of the 21,2 <— 17,1 vibration-rotation transition of the HD+ ion, obtained by Doppler tuning the ion beam into resonance with a carbon dioxide infrared laser beam [88], (b) A radiofrequency/infrared double resonance spectrum obtained by pumping the v3 line shown in the infrared spectrum (see text for the assignment). 

The hope that chemical reactions can be used to pump efficient and powerful lasers is a major stimulus to studies of the chemical production of excited species [429]. There have now been a very large number of papers [430] on lasers which oscillate on the vibration-rotation transitions of molecules produced in atom-transfer reactions, although laser oscillation following... 

D. Laughton, S. M. Freund, and T. Oka (private communication) detected for the first time two Ak = 3 forbidden vibration-rotation transitions in the I j band of NH3 using infrared microwave two-photon spectroscopy and laser Stark spectroscopy (cf. Section 4.3). This has made it possible to obtain the Co rotational constant of6.2280 +0.0008 cm"... 

At wavelengths X > 165 run, the flash photolysis of OCS excites laser emission on vibrationally excited CO(AT E ) and up to thirty vibration-rotation transitions have been identified, ranging from A (13 - 12) to Aw(7 Laser emission... 

Gas-Discharge Lasers Using Vibrational-Rotational Transitions CO2 Lasers... 

The medium infrared spectral region contains typically vibrational transitions of molecules and their rotational substructure. Therefore it is obvious, that one can use vibration rotation transitions in a laser medium itself, provided there is an inversion mechanism available. However, in the gas phase such transitions are fairly narrow and therefore will not be the ideal source for spectroscopy, where one would like to have a continuously tunable laser source in order to scan across a series of vibration-rotation transitions of the molecular gas to be investigated. Although we can make use of it for very special situations e.g.for the spectroscopy of paramagnetic molecules, where Zeeman-tuning of the molecular transition can be achieved, we must use other types of gain media for a tunable infrared laser. 

There are excellent coincidences between vibration rotation transitions of the NO fundamental in the X n and some strong CO-laser lines near 5.2 jim. If we shine a few watts of this co-laser light into an absorption cell containing NO and Ar, those y- and p-bands occur at IR-laser power densities of less than 1 kW/cm. This means that there must be a way for the energy that is put into the fundamental vibration of the diatomic molecule to get up the ladder of vibrational states to the level of electronic excitation or to the dissociation limit [3,2/3,3]. For a diatomic molecule, particularly at these low power densities, multiphoton excitation is not possible. 

The sub-Doppler nature of double resonance with single mode c.w. lasers has been used to resolve hyperfine structure in an infrared-optical study of NH2 (Amano et. al. 1982). A fluorescence cell was placed inside the cavity of a CO2/N2O laser and between the poles of a 15" electromagnet capable of fields up to 22 kG. A dye laser beam was introduced into the cell through a ZnSe mirror, and excited NH2 fluorescence through transitions to levels of the V2 = 9 or 10 states. At magnetic fields which Zeeman tuned into resonance vibration-rotation transitions within the excited state the population transfer changed... 

For the spectroscopy of vibrational-rotational transitions in molecules the laser-excited fluorescence is generally not the most sensitive tool, as was discussed at the end of Sect. 1.3.1. Optoacoustic spectroscopy, on the other hand, is based on colli-sional energy transfer and is therefore not applicable to molecular beams, where collisions are rare or even completely absent. For the infrared spectroscopy of molecules in a molecular beam therefore a new detection technique has been developed, which relies on the collision-free conditions in a beam and on the long radiative lifetimes of vibrational-rotational levels in the electronic ground state [84-86]. 

This technique was first applied to the infrared region where many vibrational-rotational transitions of ions were measured with color-center lasers or diode... 

A monochromatic laser beam is sent through a sample of diatomic molecules. The laser wavelength is tuned to a vibration-rotation transition (u", J") (v J )... 

In the infrared spectral range. Lamb dips of a vibration-rotation transition of CH4 at 1 = 3.39 pm or of CO2 around 10 pm are commonly used for frequency stabilization of the HeNe laser at 3.39 pm or the CO2 laser. In the visible range various hyperfine components of rotational lines within the -> system of the I2 molecule are mainly chosen. The experimental setup is the same as that shown in Fig. 2.18. The laser is tuned to the wanted hfs component and then the... 

One example of this pump-and-probe technique is the investigation of collision-induced vibrational-rotational transitions in the different isotopes HDCO and D2CO of formaldehyde by an infrared-UV double resonance [1041]. A CO2 laser pumpes the V6 vibration of the molecule (Fig. 8.18). The collisional transfer into other vibrational modes is monitored by the fluorescence intensity induced by a tunable UV dye laser with variable time delay. 

In order to achieve more accurate values for the ratio of electron mass to proton mass from which the fine-structure constant can be derived, a frequency comb in the infrared region was developed which can be used to measure vibrational-rotational transitions in HD+-ions. The frequency distance q between the modes of the comb, which equals the repetition rate of the femto-second pulses, is stabilized on the resonance frequency of a cryogenic ultra-stable sapphire resonator. For the measurement of the transitions in the HD+-ion diode lasers with external cavity and a grating for achieving single mode operation are used. Their frequency is stabilized onto the centre of the molecular transition and is then compared with the adjacent mode of the frequency comb. 

Example 10.1 With a diode laser spectrometer and a multipass absorption cell (Fig. 10.1) NO2 concentrations down to the 50-ppt level in air were detected on a vibrational-rotational transition at 1900 cm NO concentrations down to 300 ppt, while for SO2 at 1335 cm a sensitivity limit of 1 ppb was reached [1371]. 

For such absorption measurements infrared lasers can be used which coincide with vibrational-rotational transitions of the investigated molecules (CO2 laser, CO laser, HF or DF lasers, etc.). Particularly useful are tunable infrared lasers (diode lasers, color-center lasers, or optical parametric oscillators, Vol. 1, Sect. 5.7), which may be tuned to selected transitions. The usefulness of diode lasers has been demonstrated by many examples [1451]. For instance, with a recently-developed automated diode laser spectrometer, which is tuned by computer control through the spectral intervals of interest, up to five atmospheric trace gases can be monitored in unattended operation. Sensitivities down to 50 ppt for NO2 and 300 ppt for NO have been reported [1371]. 

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