Vibrational rotational levels

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

As a first example for the application of this technique, we mention the investigation of Stark splitting in molecules studied with a CO2 laser by Brewer etal. The authors shifted the vibration-rotation levels of by an external electric field. With increa-... 

Most of the infrared laser lines originating from transitions between vibrational-rotational levels in different electronic states of diatomic molecules, e. g. N2, Oj, H, D2, CO, CN, etc., have been meanwhile correctly indentified Some lines and term systems have been found which had never been observed before 354a)... 

To investigate the spectra of diatomic molecules, we need the selection rules for radiative transitions. We now investigate the electric-dipole selection rules for transitions between vibration-rotation levels belonging to the same 2 electronic state. (Transitions in which the electronic state changes will be considered in Chapter 7.)... 

In calculating the moment of inertia, Ie = fiR 2, one customarily evaluates ja from the masses of the neutral atoms, considering this mass to be concentrated at the nucleus.1 Of course, this is an approximation, but it is a very good one, since electrons are so light. The atomic masses ma and mb in (4.5) cannot usually be gotten from tables of atomic weights, since most elements are mixtures of different isotopes. Each isotopic species has its own vibration-rotation levels. Masses for some common isotopes are given in the Appendix. 

With each electronic state, we have a series of vibration-rotation levels. See Fig. 4.3. The total molecular energy (excluding translation) for a given state of electronic and nuclear motion is the sum of the equilibrium electronic energy Ue, the vibrational energy vib, and the rotational energy... 

In linear molecules, the electronic-rotation interaction terms in H cause the A-type doubling of electronic states, whereas the vibration-rotation interaction terms in H cause the /-type doubling of vibrational states. In addition, the perturbation H can cause interactions between vibration-rotation levels of different electronic states. If it happens that two vibration rotation levels of different electronic states of a molecule have... 

PERTURBED VIBRATIONAL-ROTATIONAL LEVELS, JAHN-TELLER-RENNER EFFECTS... 

Auzinsh, M.P. and Ferber, R.S. (1989). Optical orientation and alignment of high-lying vibrational-rotational levels of diatomic molecules under their fluorescence population, Opt. Spectrosc. (USSR), 66, 158-163. 

Here, the techniques concerned with the measurement of the kinetics and dynamics of excited states produced by laser excitation will be discussed in detail. The narrow-band tunable dye laser is very useful in these studies since the frequency of the laser can be tuned into coincidence with a particular transition in a molecule, producing a well-defined excited state. The examples which follow are principally concerned with electronic excitation to a particular vibrational—rotational level. Similar techniques can be applied to the study of vibrational distributions [34] and relaxation in a particular electronic state, the difference being that the excitation and emission wavelengths are much longer (infrared) and different lasers and detectors must be used for this spectral region. 

A particularly clear example of Hund s case (c) coupling has been observed for the HeAr+ ion in its near-dissociation vibration-rotation levels [58] it also occurs for the I2 molecule [59],... 

Notice that in both case (d) and case (e) there is no molecular projection quantum number. An example of case (e) coupling, probably the first, has been observed [60] for vibration rotation levels of the HeKr+ ion which lie very close to the dissociation limit. The Kr+ atomic ion has L = 1 and S= 1/2, so that. Ja is 3/2 or 1 /2, and the spin orbit interaction is strong. When a very weak bond is formed with a He atom,. Ja remains a good quantum number, at least for the most weakly bound levels, but there are nevertheless series ofrotation levels, with rotational energy BR(R + 1). The details are described in chapter 10, where we show that case (e) coupling is identified, both by the observed pattern of the rotational levels, and by the measured Zeeman effects and effective g factors for individual rotational levels. 

Relationships between potential functions and the vibration-rotation levels... 

Most spectroscopic studies involve the lowest energy vibration-rotation levels, and the determination of the values of the molecular parameters at or near the equilibrium position. This is equally true of most theoretical studies indeed there are many published accurate ab initio calculations of equilibrium properties which do not even extrapolate with the correct analytical form to the dissociation asymptote. Calculations which... 

A vibration rotation level of a diatomic molecule which lies above the lowest dissociation limit may be quasibound and able to undergo spontaneous dissociation into the separate atoms. This process is known as predissociation, and two different cases may be distinguished for diatomic molecules, as we will see shortly. Predissociation does not normally play an important role in rotational spectroscopy but merits a brief discussion here for the sake of completeness. 

Figure 10.33. Tandem mass spectrometer system employing electric field dissociation, designed to enable the study of microwave spectra of molecular ions involving vibration-rotation levels lying close to the dissociation limit.

---