Absorption of the Laser Radiation

The role of the optical absorption of the matrix in the transfer of energy from the laser beam to the sample is governed by Beer s law [14] 

For the MALDI process, the energy absorbed per unit volume EJV of the sample (loosely called energy density ) is the process-determining quantity. This can be derived from Eq. (1.1) by simple differentiation to  

Equation (1.2) is at the core of the MALDI process. If a matrix is chosen with a sufficiently high absorption coefficient a, a relatively low fluence Ho suffices for achieving the critical energy density necessary to initiahze ablation and ionization of a top layer of the sample. Values for Hq of 50-500 J m are representative for most UV-MALDI applications. 

As discussed above, every laser exposure of a sample leads to the removal of a bulk volume - that is, many monolayers of matrix molecules of the sample. The term desorption is, therefore, somewhat ill-chosen for this process, and was so even for the field desorption for which it was originally coined. Ablation (removal of bulk material from surfaces) is the more specific term, and is used interchangeably with desorption throughout this chapter. The processes of material ablahon and the ionizahon of a minor fraction of the matrix and analyte molecules are, no doubt, intimately intertwined, and both take place on a micrometer geometric and a nanosecond time scale. It is experimentally very difficult - if not impossible - to sort out the complex contributions of the physical processes induced by the laser irradiation in all detail. Despite this complexity, it is of considerable merit to treat the ablation and ionization mechanisms separately. From such a discussion, some basic understanding can be derived, particularly, because the vast majority of the ablated material comes off neutral. 

Eor IR-MALDI, the situation can be very different because of the larger penetration depth, resulting in a larger acoustic time constant of about 1 ns. Eor the desorption with an Er YAG laser, the pulse width of 100 ns is long compared to 

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Measurements of ozone concentration in the ozone layer in the stratosphere are made in the less intense Huggins band to avoid complete absorption of the laser radiation. Again, the two or three wavelength DIAL method is used to make allowance for background aerosol scattering. A suitable laser for these measurements is the XeCl pulsed excimer laser (see Section 9.2.8) with a wavelength of 308 nm, close to the peak absorption of the Huggins... 

This desorption ionisation technique leads to weak fragmentation. The analyte is incorporated into a solid organic matrix (such as hydroxybenzoic acid) and the mixture is placed on a sample holder that is irradiated with UV laser pulses (e.g. N2 laser, A = 337 nm, pulse width = 5 ns). The laser energy is absorbed by the matrix and transferred to the analyte, which becomes desorbed and ionised (Fig. 16.18c). Although MALDI is considered to be a soft ionisation technique, a substantial amount of energy is involved. Because the technique involves pulsed ionisation, it is well suited for time-of-flight mass analysis of biomolecules. The analysis of small molecules (M < 500 Da) is limited because the matrix decomposes upon absorption of the laser radiation. However, solid supports such as silicone can be used as the matrix to overcome this disadvantage. 

Photoablation (diode-pumped Nd YV04 laser, X = 532 nm) was used to create a master on the PMMA layer coated on a Si wafer (see Figure 2.20a). The PMMA layer was doped with rhodamine B to facilitate the absorption of the laser radiation. The width of the ablated features depends on the diameter and the position of the laser focal point. The best aspect ratios were obtained with the laser beam focused 3 1 pm into the PMMA film. The ablated PMMA-Si master was used to cast a PDMS layer (see Figure 2.19b). The cast PDMS layer appeared to have smoother surfaces than the PMMA master [367]. 

The analysis of small molecules (M < 500 Da) is limited because the matrix decomposes upon absorption of the laser radiation. However, other supports... 

An increase in the absorption of the laser radiation in the melt, as well as textural changes on the sample surface, were clearly observed in melting experiments on cubic BN at 10 GPa described below. Based on previous estimates of the triple point between hexagonal BN, cubic BN, and liquid [27], melting of cubic BN at... 

The energy level structure of the Ti " " ion is unique among transition-metal laser ions because it does not possess d-state energy levels above the upper laser level. This eliminates excited-state absorption of the laser radiation, giving Tirsapphire the observed wide tuning range and great efficiency. 

The temperature field can be calculated on the basis of the general differential equation of thermal conduction. The general differential equation of thermal conduction describes the non-steady-state temperature field, with the absorption of the laser radiation being taken into account as an internal heat source d> ... 

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