Absorption happens when photons are absorbed by the atom and its electrons are promoted to higher energy levels. Absorption can be studied measuring the amount of laser light transmitted by the sample as a function of the wavelength. Fluorescence happens when the excited electron decays back to its original state by emitting a photon of light at a longer wavelength (smaller energy) than the exciting photon from the laser. Scattering may be elastic (like Rayleigh and Mie scattering) or inelastic. Inelastic scattering takes places in atoms, molecules or lattices where part of the photon energy is transferred to/from the medium in processes like Raman or Brillouin scattering. In this section we consider mostly absorption and fluorescence spectroscopy and steady-state phenomena with high spectral resolution, rather than time-resolved studies, where pico- or femtosecond pulses are used. For a description of Raman scattering applications, please go to the section “Raman Spectroscopy”.
There are many high-resolution spectroscopic techniques. For example, the laser wavelength can be tuned to an electronic absorption of the sample and the resulting fluorescence can be analyzed as a function of wavelength. Alternatively, if laser excitation at an initial wavelength is introduced to a sample, absorption/transmission spectroscopy can be used to monitor the amount of light transmitted through the sample. If the excitation wavelength is scanned, an absorption spectrum can be analyzed. Many distinctions of this spectroscopy exist.
Lasers play an important role in high-resolution spectroscopy because of their simultaneous high-power output beam and narrow linewidth. Atomic spectroscopy (both absorption and fluorescence) usually requires wavelengths between 100 nm and 2 micron. Tunable Titanium Sapphire lasers are ideally suited for atomic spectroscopy because they provide powers and tuning ranges unavailable with any other source. The natural tuning range of 700-1,000 nm can be extended using second harmonic generation to cover the region 350-500 nm. Coherent’s MBR family of CW tunable lasers offer all these wavelength ranges with different power levels and with various linewidths. The flagship model MBR-110 provides a very narrow 75 kHz linewidth, suitable for high-resolution studies. The laser output of the MBR lasers can be stabilized and scanned to provide accurate spectra of atomic species. The MBD-200 resonant ring frequency-doubler efficiently converts the MBR output to its second harmonic and further extends the available wavelength range.
Coherent’s Ion lasers (Innova 300 series) can be operated in single frequency at several blue and green wavelengths and provide linewidths narrower than 50 MHz. The single frequency Verdi DPSS laser produces up to 18 Watts at 532 nm and 20 W at 1064 nm. The specifically designed doubler MBD-266 can be used to convert the green output of Verdi to its second harmonic in the deep UV.