Molecular spectroscopy

rotational and vibrational spectra

The origin of spectral lines in molecular spectroscopy is the absorption, emission, or

scattering of a photon when the energy of a molecule changes. The difference from

atomic spectroscopy is that the energy of a molecule can change not only as a result of

electronic transitions but also because it can undergo changes of rotational and

vibrational state. Molecular spectra are therefore more complex than atomic spectra.

However, they also contain information relating to more properties, and their analysis

leads to values of bond strengths, lengths, and angles. They also provide a way of

determining a variety of molecular properties, particularly molecular dimensions,

shapes, and dipole moments. Molecular spectroscopy is also useful to astrophysicists

and environmental scientists, for the chemical composition of interstellar space and of

planetary atmospheres can be inferred from their rotational, vibrational, and electronic

spectra. Pure rotational spectra, in which only the rotational state of a molecule

changes, can be observed in the gas phase. Vibrational spectra of gaseous samples show

features that arise from rotational transitions that accompany the excitation of

vibration. Electronic spectra, show features arising from simultaneous vibrational and

rotational transitions. The simplest way of dealing with these complexities is to tackle

each type of transition in turn, and then to see how simultaneous changes affect the

appearance of spectra.

General features of spectroscopy All types of spectra have some features in common, and we examine these first.

In

emission spectroscopy, a molecule undergoes a transition from a state of high

energy EI to a state of lower energy E 2 and emits the excess energy as a photon.

In absorption spectroscopy, the net absorption of nearly monochromatic (single

frequency) incident radiation is monitored as the radiation is swept over a range

of frequencies. We say net absorption, because it will become clear that, when a

sample is irradiated, both absorption and emission at a given frequency are

stimulated, and the detector measures the difference, the net absorption.

The energy, hν, of the photon emitted or absorbed, and therefore the

frequency

ν of the radiation emitted or absorbed, is given by the Bohr frequency condition,

hv = │ EI - E2 │. Emission and absorption spectroscopy give the same

information about energy level separations, but practical considerations generally

determine which technique is employed. We shall discuss emission

spectroscopy; here we focus on absorption spectroscopy, which is widely

employed in studies of electronic transitions, molecular rotations, and molecular

vibrations.

Previously we saw that transitions between electronic energy levels are

stimulated by or emit ultraviolet, visible, or near-infrared radiation. Vibrational and

rotational transitions, the focus of the discussion in this chapter, can be induced

in two ways. First, the direct absorption or emission of infrared radiation can

cause changes in vibrational energy levels, whereas absorption or emission of

microwave radiation gives information about rotational energy levels. Second,

vibrational and rotational energy levels can be explored by examining the

frequencies present in the radiation scattered by molecules in Raman

spectroscopy. About 1 in 10 7 of the incident photons collide with the molecules,

give up some of their energy, and emerge with a lower energy. These scattered

photons constitute the lower-frequency Stokes radiation from the sample (Fig.1).

Other incident photons may collect energy from the molecules (if they are already

excited), and emerge as higher-frequency anti-Stokes radiation. The component

of radiation scattered without change of frequency is called Rayleigh radiation.

Fig. 1 In Raman spectroscopy, an incident photon is scattered from a molecule with either an increase in frequency (if the radiation collects energy

from the molecule) or-as shown here for the case of scattered Stokes radiation-with

a lower frequency if it loses energy to the molecule. The process can be regarded as

taking place by an excitation of the molecule to a wide range of states

(represented by the shaded band), and the subsequent return of the molecule to a

lower state; the net energy change is then carried away by the photon.

Experimental techniques A spectrometer is an instrument that detects the characteristics of light scattered,

emitted, or absorbed by atoms and molecules. Figure 2 shows the general

layouts of absorption and emission spectrometers operating in the ultraviolet and

visible ranges. Radiation from an appropriate source is directed toward a sample.

In most

Fig. 2 Two examples of spectrometers: (a) the layout of an absorption

spectrometer, used primarily for studies in the ultraviolet and visible ranges, in which

the exciting beams of radiation pass alternately through a sample and a

reference cell, and the detector is synchronized with them so that the relative

absorption can be determined, and (b) a simple emission spectrometer, where light

emitted or scattered by the sample is detected at right angles to the direction of

propagation of an incident beam of radiation.

Spectrometers, light transmitted, emitted, or scattered by the sample is collected

bymirrors or lenses and strikes a dispersing element that separates radiation into

different frequencies. The intensity of light at each frequency is then analysed by

a suitable detector. In a typical Raman spectroscopy experiment, a

monochromatic incident laser beam is passed through the sample and the

radiation scattered from the front face of the sample is monitored (Fig. 3). This

detection geometry allows for the study of gases, pure liquids, solutions,

suspensions, and solids. Modern spectrometers, particularly those operating in

the infrared and near-infrared, now almost always use Fourier transform

techniques of spectral detection and analysis. The heart of a Fourier transform

spectrometer is a Michelson inter-ferometer, a device for analysing the

frequencies present in a composite signal. The total signal from a sample is like a

chord played on a piano, and the Fourier transform of the signal is equivalent to

the separation of the chord into its individual notes, its spectrum.

Fig. 3 A common arrangement adopted in Raman spectroscopy. A laser beam first passes through a lens and then through a small hole in a mirror with a curved reflecting surface. The focused beam strikes the sample and scattered light is both deflected and focused by the mirror. The spectrum is analysed by a monochromator or an interferometer.