Physical Chemistry Third Edition

23.1 Emission/Absorption Spectroscopy and Energy Levels 955

410.2 (violet)434.0 (violet)486.1 (green-blue) 656.3 (red)

400 500 600 700

/nm

Figure 23.2 The Visible Portion of the Hydrogen Atom Emission Spectrum

(Simulated).Each wavelength represented produces an image of the slit of the spec-

trograph. If only discrete wavelengths are present, as in this case, the spectrum is called a

line spectrum.

Toroid

mirror

Toroid

mirror

Paraboloid

mirror

Dual grating

(on turntable)

Thermocouple

Filter Exit slit

Ellipsoid

mirror

Entrance slit

Plane mirror

Plane

mirrors

Optical wedge

reference beam

Sampling area

Sample beam

100% comb

Plane mirror

Toroid

mirror

Aperture

stop

Sector

mirror

Plane

mirrors

Source

Figure 23.3 Schematic Diagram of a Filter-Grating, Double-Beam Infrared Spec-

trophotometer.This diagram shows a scanning instrument. The wavelength passed is

determined by the angular position of the grating, which is mounted on a turntable, and is

rotated automatically by a motor. Courtesy of Perkin-Elmer Corporation.

Absorption Spectroscopy

Absorption spectroscopy has traditionally been carried out in aspectrophotometer,

such as the one shown schematically in Figure 23.3. The light is dispersed by a prism

or grating, collimated into a beam of nearly parallel rays, and passed through a cell

containing the sample. Only a narrow band of wavelengths passes at one time and the

bandwidth determines the resolution of the instrument. The wavelength can be chosen

by turning the prism or grating, and this is done automatically in a scanning instrument.