Molecular Spectroscopy by Laser-Induced Fluorescence

Assume a rovibronic level (n, 7 ) in an excited electronic state of a diatomic molecule has been selectively populated by optical pumping. With a mean Ufe-time T = the excited molecules undergo spontaneous transitions to 

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

The spontaneous transition probability is proportional to the square of the matrix element (Vol. 1, Sect. 2.7.2) 

Only those transitions for which all three factors are nonzero appear as lines in the fluorescence spectrum. The Honl-London factor is always zero unless 

If a single upper level (n, Jj ) has been selectively excited, each vibrational band consists of at most f/tree lines SiPline AJ = -1), a Q/me (A/ = 0), and an R line (A/ = - -1). For diatomic homonuclear molecules additional symmetry selection rules may further reduce the number of possible transitions. A selectively excited level v f., J ) in a /7 state, for example, emits on a 77 i transition either 

As has been shown in Sect.2.9, the total wave function can be separated into a product 

If a single upper level (V Jj ) has been selectively excited, each vibrational 

Assume a rovibronic level (v, J0 in an excited electronic state of a diatomic molecule has been selectively populated by optical pumping. With a mean lifetime fjj = l/S Ajjn, the excited molecules undergo spontaneous transitions to lower levels E (v ,J ) (Fig.6.33). At a population density N (v, J0 the radiation power of a fluorescence line with frequency -(Eij-E )/h is given by (Sect.2.6) 

Assume a rovibronic level in an excited electronic state of a di- 

Akm OC MelP MvibP MrotP, where the first factor 

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Volume n/24 presents the spectroscopic data on diamagnetic and paramagnetic molecules as well as on molecular ions and radicals up to date considering the publications up to and partly including 1997. The spectroscopic information collected in this volume has been obtained principally from gas phase microwave measurements. In addition, gas phase data have been included derived from methods related to microwave spectroscopy by employing a coherent radiation source. These are molecular beam techniques, radio frequency spectroscopy, electron resonance spectroscopy, laser spectroscopy, double resonance and saturation techniques. Some other methods are considered if the accuracy of the derived molecular parameters is comparable to that of microwave spectroscopy owing to a good statistics in the analysis of data, and no microwave data are available. Examples would be Fourier infrared spectroscopy or laser induced fluorescence. 

As we will see the use of saturated laser induced fluorescence spectroscopy will allow us to ignore some of these effects. we can infer the importance of others and for the time being the remainder have to be evaluated on a case by case basis for each molecular system and for the operating parameters of the experiments. 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

In the columns identifying the experimental method, MW stands for any method studying the pure rotational spectrum of a molecule except for rotational Raman spectroscopy marked by the rot. Raman entry. FUR stands for Fourier transform infhired spectroscopy, IR laser for any infiured laser system (diode laser, difference frequency laser or other). LIF indicates laser induced fluorescence usually in the visible or ultraviolet region of the spectrum, joint marks a few selected cases where spectroscopic and diffraction data were used to determine the molecular structure. A method enclosed in parentheses means that the structure has been derived from data that were collected by this method in earlier publications. The type of structure determined is shown by the symbols identifying the various methods discussed in section II. V/ refers to determinations using the Kraitchman/Chutjian expressions or least squares methods fitting only isotopic differences of principal or planar moments (with or without first... 

Wang, Y. Hendrickson, C.L. Marshall, A.G. Direct optical spectroscopy of gas-phase molecular ions trapped and mass-selected by ion cyclotron resonance Laser-induced fluorescence excitation spectrum of hexafluorobenzene (C F ). J. Chem.Phys. Lett. 2001,334, 69-75. 

A major advance in the utility of laser spectroscopy came as a result of the development of multiphoton ionization MPI as a means of detection of multiphoton absorption by molecules [1]. The resonance encountered as the n-photon energy of a scanning laser becomes coincident with that of a molecular excited state is evidenced by a large increase in ionization rate. Since single ionization events can be detected with near unit efficiency, this results in a very sensitive means of detecting weak multiphoton absorption. MPI is a more widely applicable method than laser induced fluorescence since it can be used for non-emitting states. 

A typical laser spectrometer for sub-Doppler excitation spectroscopy in a collimated molecular beam is shown in Fig. 4.2. The laser wavelength Xl is controlled by a computer, which also records the laser-induced fluorescence /fK l). Spectral regions in the UV can be covered by frequency-doubling the visible laser frequency... 

In the molecular beam, species selective absorption spectroscopy of a-Naphtol (NH3)n clusters was performed by one-colour resonant two-photon-lonlzatlon (R2P1). Based on the Identification of the spectral features of each complex, the laser Induced fluorescence (LIF) emission spectra were then measured as a function of the cluster size. The search for proton transfer in these clusters Involved the monitoring of fluorescence emission from the undissociated a-Naphtol and/or the Naphtolate anion. 

Using space-time-resolved laser-induced fluorescence and plasma-induced emission spectroscopy, the interaction between BCI3 and Ar gives metastable and dissociation product densities that vary nonlinearly as Ar is diluted by BCI3. A model is proposed in which argon metastable states indirectly enhance molecular dissociation [7]. 

In the previous chapter we have seen how tunable lasers can be used in a multitude of ways to gain basic information on atomic and molecular systems. Thus, the laser has had a considerable impact on basic research, and its utility within the applied spectroscopic field is not smaller. We shall here discuss some applications of considerable interest. Previously, we have mainly chosen atomic spectroscopic examples rather than molecular ones, but in this chapter we shall mainly discuss applied molecular spectroscopy. First we will describe diagnostics of combustion processes and then discuss atmospheric monitoring by laser techniques. Different aspects of laser-induced fluorescence in liquids and solids will be considered with examples from the environmental, industrial and medical fields. We will also describe laser-induced chemical processes and isotope separation with lasers. Finally, spectroscopic aspects of lasers in medicine will be discussed. Applied aspects of laser spectroscopy have been covered in [10.1,2]. 

The occurrence of proton transfer, i.e., of proper Bronsted acidity, is usually revealed by IR spectroscopy, because the vibrational modes of protonated and non-protonated species are remarkably different. This topic, now a standard tool in surface chemistry, is dealt with in detail in other chapters of the present volume, and will not be treated here (cf. also Volmne IV, Chapter 1 of the present series Molecular Sieves - Science and Technology ). A variant of the IR method is the use of quinoline instead of pyridine, which allows the distinction between protonated and non-protonated species by the use of laser-induced fluorescence [32]. 

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