The 248-nm Excimer-Laser-Ablation Mechanism of Liquid Benzene Derivatives:  Photochemical Formation of Benzyl Radical Leads to Ablation

Abstract

The mechanism underlying 248-nm laser ablation of liquid benzene derivatives (alkyl benzenes, benzyl chloride, benzyl alcohol) was revealed by means of transient absorption spectroscopy. One characteristic in the present liquid system is that the ablation threshold can be correlated not to the boiling point at all but to the photochemical reactivity of β-bond cleavage. In the spectroscopic measurement, the benzyl-radical formation was confirmed upon ablation, and its concentration was quantitatively evaluated at the threshold. The obtained value was ∼0.05 M, which was almost common to all of the examined liquids. The result means that the present liquids, whose macroscopic physical properties such as surface tension do not differ too much from each other because of the analogous molecular structure, undergo the ablation when the radical concentration reaches the critical value of 0.05 M. This is the first demonstration for bridging a gap between microscopic photochemical process and macroscopic morphological change. The ablation behavior can be well interpreted in terms of the photochemical volume explosion.

The role of the photochemical fragmentation in laser ablation: a molecular dynamics study

“…The reactions also affect the ablation threshold value that is lower in the system with photochemistry as compared to the system with vibrational relaxation only. Indeed, it was discussed by Tsuboi et al [1,2] that, in the ablation of the liquid chlorobenzene sample the estimated temperature at the ablation threshold (97 • C) is below the boiling point (132 • C) signifying the contribution of photochemical fragmentation processes. Conversely, for benzene in which only vibrational relaxation of the molecules occurs, the estimated temperature is higher than the boiling point.…”

“…Tsuboi et al [1,2] have reported on 248 nm laser ablation of liquid benzene and benzene derivatives such as toluene, benzyl chloride, chlorobenzene and benzyl alcohol. Based on the values of temperature estimated for the threshold fluence, they concluded that the ablation of benzene is induced by explosive boiling of the overheated liquid.…”

“…Photofragmentation of chlorobenzene occurs exclusively by scission of the C-Cl bond to yield C 6 H 5 and Cl radicals, which in solution and static gas cell experiments react with the precursor molecules to form a number of different products [10]. The photochemistry of C 6 H 5 Cl has been investigated by numerous experimental techniques [1,2,9,[11][12][13][14]25]. Using a C 6 H 5 Cl molecular beam [13], it was found that excitation of chlorobenzene by 248 nm laser light can lead to either photochemical dissociation or vibrational relaxation.…”

Computer Simulations of Laser Ablation of Molecular Substrates

“…This specific percentage has been observed experimentally for 248-nm irradiation of a molecular beam of chlorobenzene. 112 The dependence of the amount of material removed per laser pulse versus fluence for these two systems is shown in Figure 14. Regardless of the presence of photochemistry, both systems exhibit two distinct mechanisms of ejection, desorption, and ablation, separated by the ablation threshold.…”

Laser-Induced Backside Wet Etching of Transparent Materials with Organic and Metallic Absorbers

“…Though the threshold depends on the liquid absorption, there is no linear correlation as the pyrene doping causes a sevenfold increase of the absorption but the threshold is reduced only little. The laser-induced decomposition of halogenated hydrocarbons, however, can result in halogen radicals [57,58,60,61] and can, therefore, contribute to the etching [58,62]. However, this does not explain the lower etch rate if at halogenated organics the same processes occur.…”