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Spectroscopy
CH002.qxd
9/1/2007
11:40 AM
Page 59
–2– Spectroscopy
GENERAL FEATURES In this chapter, we consider resonant interactions of biomolecular systems at first with photons and next with electrons. The different spectroscopic methods using either tunable photon or electron sources are treated independently, as generally done, in this chapter. It is nevertheless worthwhile to note that they present common features as will be shown in Chapter 4.7. Except for the terahertz region which only begins to emerge, a very sizeable fraction of the lowest energy part of the electromagnetic spectrum, ranging from the microwave region up to the vacuum ultraviolet (VUV) region is already widely used for studying biomolecular systems in the gas-phase. In the case of electrons (see Section 2.3), this range corresponds to the so-called sub-thermal and thermal energy range (0–10 eV). The electromagnetic domain will be first considered in the three spectral regions corresponding respectively to quantization of the rotation energy (molecules are considered as rigid rotors), the vibrational energy (molecules are considered as sums of anharmonic oscillators) and the electronic energy. Transitions between rotational and vibrational states respectively occur in the microwave (Section 2.1.2) and infrared (IR) (Section 2.1.3) spectral regions while transitions between electronic states take place in the visible/UV and VUV (Section 2.1.4) regions (Figure 2.1.1). Different experimental methods are used to explore those regions and will be considered in the next paragraphs. Some of these methods such as IR/UV, IR/VUV or rotationally resolved spectroscopy simultaneously involve different spectral ranges.
3 1010
3 1011
3 1012
microwave
Tz
3 1013 far-
microwave sources klystrons, carcinotrons, Thz 100
101 rotations
102 collective motions
3 1014
mid-
near
FEL 103
3 1015
lasers 104
vibrations
visible
Hz UV
synchrotrons 105
cm−1 electronic excitation
Figure 2.1.1 Electromagnetic spectrum, tunable sources, transition frequencies and energies. 59
9/1/2007
11:40 AM
Page 59
–2– Spectroscopy
GENERAL FEATURES In this chapter, we consider resonant interactions of biomolecular systems at first with photons and next with electrons. The different spectroscopic methods using either tunable photon or electron sources are treated independently, as generally done, in this chapter. It is nevertheless worthwhile to note that they present common features as will be shown in Chapter 4.7. Except for the terahertz region which only begins to emerge, a very sizeable fraction of the lowest energy part of the electromagnetic spectrum, ranging from the microwave region up to the vacuum ultraviolet (VUV) region is already widely used for studying biomolecular systems in the gas-phase. In the case of electrons (see Section 2.3), this range corresponds to the so-called sub-thermal and thermal energy range (0–10 eV). The electromagnetic domain will be first considered in the three spectral regions corresponding respectively to quantization of the rotation energy (molecules are considered as rigid rotors), the vibrational energy (molecules are considered as sums of anharmonic oscillators) and the electronic energy. Transitions between rotational and vibrational states respectively occur in the microwave (Section 2.1.2) and infrared (IR) (Section 2.1.3) spectral regions while transitions between electronic states take place in the visible/UV and VUV (Section 2.1.4) regions (Figure 2.1.1). Different experimental methods are used to explore those regions and will be considered in the next paragraphs. Some of these methods such as IR/UV, IR/VUV or rotationally resolved spectroscopy simultaneously involve different spectral ranges.
3 1010
3 1011
3 1012
microwave
Tz
3 1013 far-
microwave sources klystrons, carcinotrons, Thz 100
101 rotations
102 collective motions
3 1014
mid-
near
FEL 103
3 1015
lasers 104
vibrations
visible
Hz UV
synchrotrons 105
cm−1 electronic excitation
Figure 2.1.1 Electromagnetic spectrum, tunable sources, transition frequencies and energies. 59