Line Light Sources

Doppler broadening is an example of inhomogeneous broadening, in which different atoms contribute to different parts of the line profile. Light of frequency Vq, emitted from a source moving with a thermal velocity v, towards (v 0) or away (v 0) from an observer is recorded with a frequency shift Ai given by 

In a light source or in an absorption cell the velocities of the atoms follow a Maxwell distribution and the number of atoms with velocity components between v and v+dv in the direction of observation is 

Here N is the total number of atoms, M is the atomic weight and R is the general gas constant. The light power emitted in the frequency interval v to 

The full width at half maximum (FWHM), the Doppler width, is then 

A general rule, which may easily be memorized, is that the typical Doppler width in the visible range for typical light sources is about 0.001 nm or 1000 MHz. In the IR region the width is, of course, proportionally smaller, i.e. about 50MHz at lO/xm. 

Here N is the total number of atoms, M is the atomic weight and R is the general gas constant. The light power emitted in the frequency interval u to V - - Au is denoted by Pj, du. This power is proportional to the number of light-emitting atoms in this band, i.e., the atoms with a velocity component v between v and u + du, hence 

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Thus time-resolved experiments can be performed [6.51,52]. Data for a number of facilities producing synchrotron radiation are given in Table 6.1. With synchrotrons of resonable sizes the intensity per Doppler width that is achievable is comparable to that which is obtained from efficent line light sources. However, the intensity increases towards the extreme UV (XUV) and X-ray regions, where no comparable continuum light sources exist. 

Fig.6.58. Comparison between the use of a continuum and a line light source in atomic absorption measurements...

A comparison is made in Table 9.1 between a conventional line light source and a continuous single-mode dye laser both sources with representative data. In many spectroscopic experiments a decisive factor is by what power per unit area and spectral interval a sample can be irradiated. The designation I(i ) is used for this power density/frequency unit. In the comparison in... 

In contrast to spectrophotometry, hght-scattering experiments are generally conducted at constant wavelength. Mercury vapor lamps are the most widely used light sources, since the strong lines at 436 and 546 nm are readily isolated by filters to allow monochromatic illumination. Polarizing filters are also included for both the incident and scattered beams so that depolarization can... 

Because of the narrow line width, absorption of laser energy can excite one specific state in an atom or molecule. The laser is tuned so that its wavelength matches an absorption corresponding to the desired state, which may be an electronic state or vibrational state. Absorption of laser energy can lead to excitation of specified states much more effectively than absorption of light from conventional light sources. 

Ultraviolet light sources are based on the mercury vapor arc. The mercury is enclosed ia a quart2 tube and a potential is appHed to electrodes at either end of the tube. The electrodes can be of iron, tungsten, or other metals and the pressure ia a mercury vapor lamp may range from less than 0.1 to >1 MPa (<1 to >10 atm). As the mercury pressure and lamp operating temperatures are iacreased, the radiation becomes more iatense and the width of the emission lines iacreases (17). 

National Synchrotron Light Source User s Manual Guide to the VUVandX-Ray Beam Lines. (N. F. Gmur ed.) BNL informal report no. 45764, 1991. 

Tonner et al. have taken scanning XPS microscopies at the Advanced Light Source Synchrotron Radiation Center of Lawrence Berkeley National Laboratory [2.6]. They investigated a polished and sputter-cleaned surface of mineral ilmenite with the nominal composition FeTi03, and used the Fe 3p and Ti 3p lines for imaging. Using synchrotron radiation they demonstrated spatial resolution of approximately 0.25 p,m. 

Figure 15-12. Spectrally resolved pliotocurrent of a ITO/PPV/Mg photodiode at dilTcrcnt bias after correction for dark current, light source, and monochromator response, and normalization to the same peak value. The broken line is the normalized absorption spectrum of PPV (reproduced by permission of the Institute of Physics from Ref. 143)).

In practice, the stepper consists of a machine incorporating a light source, a photomask holder and a lens for focusing the pattern on the photoresist layer on the silicon wafer. The pattern is repeated clear across the wafer, step by step, hence the name. The lens system must be of highest quality so that definition of lines and areas remain accurate and do not overlap each other. 

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