Laser beam diameter

High spatial resolution (laser beam diameter 1 p,m)... 

Mass spectrometric measurements of ions desorbed/ionized from a surface by a laser beam was first performed in 1963 by Honig and Woolston [151], who utilized a pulsed mby laser with 50 p,s pulse length. Hillenkamp et al. used microscope optics to focus the laser beam diameter to 0.5 p,m [152], allowing for surface analysis with high spatial resolution. In 1978 Posthumus et al. [153] demonstrated that laser desorption /ionization (LDI, also commonly referred to as laser ionization or laser ablation) could produce spectra of nonvolatile compounds with mass > 1 kDa. For a detailed review of the early development of LDI, see Reference 154. There is no principal difference between an LDI source and a MALDI source, which is described in detail in Section 2.1.22 In LDI no particular sample preparation is required (contrary to... 

Table 4. Tabulation of A Mg vs. A O for various chondrite components. A O laser ablation data are shown explicitly to demonstrate that averages are reflective of the bulk objects. Where data are few, the objects are small in comparison to laser beam diameter.

The small laser beam diameter, its low divergence and the possibility of focusing the beam onto an area of less than 10" cm allows small samples to be used. Schrader and Meier recorded, for instance, Raman spectra from 5 ul of acetyl-a-oxypropionitrile and from 5 pi CCI4. 

With LA-ICP-ToF-MS, using the 193 nm ArF laser (laser energy lOOmJ at 120(xm laser beam diameter), a depth resolution of 200 nm per laser shot was measured.121 LA-ICP-MS was utilized for depth profiling of copper coatings on steel with certified copper coating thicknesses from about... 

Photopolymerization of trimethylolpropane triacrylate/5% vinyl pyrrolidone. Eosin lactone 1.3 x 10-4 M, triethanolamine 0.12 M 2lrr = 514nm Ar+ laser, beam diameter = 1.4 mm. 

CH, CN Investigations. In the previous studies of CH and CN at our laboratory (41), the saturated fluorescence values were a factor of two and five respectively below those determined by absorption. If anything, the absorption measurements may have been in error to the low side. The discrepancy, it was believed, was due to the focussed laser beam diameter exceeding the radical production region. This caused an overestimate of the fluorescence sample volume, i.e. assumed to be the laser volume,and an underestimate of the species number density. Recently these experiments have been repeated using laser-pumped dye lasers which have a pulse width of 5-10 (10-9) sec and a 10 Hz repetition rate. 

Spatial resolution was limited by the 2 mm laser beam diameter. Average absorption measurements were obtained by using a 4.0 s time constant on the lock-in detector. 

The spatial resolution is determined by either the laser spot size or the collection optics, both of which are ultimately limited by diffraction. The laser beam diameter at the focal spot (twice the beam waist, wq) was discussed in Chapter 6, and is given by Eq. (6.2) ... 

We have applied the method outlined above to the selective generation and study of polarization moments of 87Rb atoms contained in a vapor cell with antirelaxation coating. Our experimental setup is shown in Fig. 8 [Yashchuk 2002], The cell is placed between a polarizer and an analyzer oriented at 45° with respect to each other and contains an isotopically enriched sample of 87Rb atoms with a density 7.109 cm-3 at 20°C. The central laser frequency is tuned near various hyperfine structure components of the D line. The typical light power is a few hundred // W and the laser beam diameter is 3 mm. The laser frequency is modulated at Q,/(27r) from 50 Hz to 1 kHz, and the frequency modulation amplitude is 40 MHz (peak to peak). The vapor cell is... 

In the setup shown in Fig. 5.11, right, fluoreseenee from the lens and the di-chroic mirror ean eause problems. Moreover, exeitation light scattered at the di-chroic mirror and reflected at the lens can cause false prepulses when the IRF has to be reeorded. These problems can be largely avoided by using a small laser beam diameter, a small mirror instead of the dichroie beamsplitter, and a hole in the centre of Lens 1. 

Figure 3.14 Left panel Helium level scheme and the relevant couplings. The dressing laser (>-5 = 51 064 nm) couples the excited state 4s 5o to the ionization continuum. The induced continuum structure is probed (at>. = 5294 nm) through ionization of the 2s So state. Right panel Variations of the ionization cross section probed with a weak probe pulse, the frequency of which is tuned across the two-photon resonance between the 2s So and 4s 5q states. The axis of the probe laser beam (diameter 0.5 mm) coincides with the axis of the dressing laser beam (diameter 3.5 mm). There is no time delay between the pulses. The laser intensities are Ip = 4 MW/cm and Ip, = 75 MW/cm. The experimental profile (dots) is in good agreement with the result from numerical studies (dotted line), which include averaging over fluctuating laser intensities and integrating over the spatial profile of the probe laser. Taken from Ref. [69].

A lens of 20 cm focal length is used to focus the laser beam on the emitter. With this the original laser beam diameter of 1.5 mm can be condensed to produce a hot... 

Dispersive Raman spectrometers are used with excitation in the visible range (typically He—Ne or Ar+ lasers are used), Fourier transform Raman spectrometers are used with excitation in the near infrared range (Nd YAG laser). For both ranges, microscopic techniques working with a laser beam diameter of micrometer size, are commercially available. 

FIGURE 7.15 Background-subtracted fluorescence intensity (counts per second) vi number of ions of the fluorescent dye, Rhodamine 640, in the laser interaction volume. The laser beam diameter is 220 (xm (fwhm). The inset shows details for < 20 ions (dashed box). (From lavarone, A.T. Duft, D. Parks, J.H. J. Phys. Chem. 2006,110,12714-12727. With permission.)... 

Since such small splittings can only be observed if the width of the Lamb peaks is smaller than the recoil shift, all possible broadening effects, such as pressure broadening and transit-time broadening, must be carefully minimized. This can be achieved in experiments at low pressures and with expanded laser beam diameters [1117], An experimental example is displayed in Fig. 9.3. 

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