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 radiationwith
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.