. The goal of this study is an in vitro evaluation of thermal side-effects by the application of short sub-µs CO 2 laser pulses in combination with an air–water spray on different types of bone tissue. A mechanically Q-switched CO 2 laser delivered 300 ns pulses at 9.6 µm wavelength, which were focused down to a spot size of 440 µm on the tissue (a corresponding energy density of 9 J/cm 2 ). Bone samples (blocks from pig femur, rib, or cartilage) were moved through the beam repeatedly until 1–5 mm deep cuts were produced. An air driven water spray was applied to prevent the tissue dehydration. Subsequent visual and histological examinations revealed no carbonisation, melting traces or fissuring of the tissue. An extremely narrow, 2–6 µm thick thermally altered layer was observed at the cut border in compacta and cartilage. No accumulation of the thermal damage occurred with increasing cut depth. Laser incisions in trabecular tissue were accompanied with a 100–200 µm thick zone of thermal necrosis in bone marrow. The difference from compacta and cartilage can be explained considering the particular character of the spreading of the ablation products in the trabecular meshwork. Minor thermal side effects make the Q-switched and probably other short pulsed CO 2 laser systems interesting for hard tissue surgery.

Isotopically selective multiphoton dissociation of CHClF2 produced a mixture of C-12/C-13 isotopomeric tetrafluoroethylenes. Specific NMR parameters for each isotopomer and the composition of this mixture were obtained from C-13 and F-19 NMR spectra by spectral analysis using consecutively lineshape and peak top iteration methods. Copyright (C) 2001 John Wiley Sons, Ltd.

The Er:YAG and the CO2 laser are competitors in the field of hard tissue ablation. The use of Er:YAG lasers (2.94 μm, pulse length τL of 100 to 200 μs) show smaller areas of thermal defects then ‘‘superpulsed’’ CO2 lasers with pulse lengths of approximately 100 μs. Only the development of a Q-switched CO2 laser (9.6 μm, τL=250 ns) allowed for similar results. In this paper new results for the Er:YAG and the Q-switched CO2 laser under the influence of water spray will be presented. Several parameters are of special interest for these investigations: the specific ablation energy, which shows a minimum for the CO2 laser at an energy density of 9 J/cm 2 and a broad shallow minimum in the range of 10 to 70 J/cm2 for the Er:YAG laser, and comparison of the cut-shape and depth. Surface effects and cutting velocity are discussed based on SEM pictures.

2-5-mm deep incisions in bone tissue were produced in vitro with 250-ns pulses of a mechanically Q- switched CO2 laser. Application of an air-water spray during irradiation prevented tissue parching. Microscopy reveals no charring or other visible defects. Histological examination shows a basophilic zone a few µm thick next to the cut surface (probably carbonization). Such a thin thermally altered layer immediately at the cut surface indicates high efficiency of the laser ablation under the experimental conditions. Coagulation necrosis of hematopoietic marrow by cutting through trabecular tissue is 30-150 µm thick. A zone of loose textured vacuolar necrosis is observed in the trabecular tissue down to a depth of 750 µm.

Using short 300 ns pulses of a Q-switch CO2 laser at the wavelength of maximal hard tissue absorption (9.6 micrometer) we achieve charring-free ablation of bone tissue. An air-water spray prevents tissue parching and helps to avoid excessive rest-heat accumulation. The observed thermally altered layer at the cut surface in a cortical bone is of only 2 - 6 micrometer thickness. A long enough irradiation results in a progressively narrowing to the bottom wedge-shaped cut profile and in a drop of the ablation rate as compared to its initial value. The main reason for this is most likely an enhancement of a heat dispersion and growing light absorption by ablation products. An enlargement of the cut width helps to avoid these negative phenomena. Using this technique we overcome restriction on the cut depth and reduce noticeably the cut time. With 66 W of average power from the Q-switch CO2 laser we need 1/2 min to produce a 6-mm deep and 10-mm long in vitro incision in a hard cortical bone (young bull femur). That time increases to 2 in for 10-mm depth.

The dynamics of infrared (IR) laser-induced pyrolysis of CF2ClCH3 has been studied by using a TEA CO2 laser radiation which consisted of a main pulse of 100 ns followed by a tail of 7 μs containing 70% of total energy. The minimum laser fluence for the effective decomposition of CF2ClCH3 is about 1.5 J cm−2 at 10P12 laser line (951.19 cm−1). Over a moderate range of laser fluence 1.5–7 J cm−2, the detected product was CF2CH2 generated via the elimination reaction of HCl from CF2ClCH3 and there was no evidence to support HF elimination from CF2ClCH3. The cross sections of photon absorption of CF2ClCH3 are measured as a function of laser fluence. The dependence of decomposition probability on pressure shows collisional dissociation at lower pressure and thermal reaction characteristics at high pressure. A model of rate equation has been used to calculate the population distributions of vibrational levels and decomposition probabilities for three kinds of laser pulse durations, which are in accord with the measured experimental results. The model calculation indicates that long laser pulse gives a wide population at the lower vibrational levels and leads to a relatively low decomposition probability.

Bone ablation with 400 ns pulses of a mechanically Q- switched CO2 laser is reported. A miniature water spray was used, which alleviates tissue carbonization, even at high laser pulse repetition rates, and increases ablation efficiency. An ablation threshold of less than 2 J/cm2, an optimal energy density of 10 J/cm2, and a corresponding specific ablation energy of 25 - 30 J/mm3 was found for pig thighbone compacta at (lambda) equals 9.57 micrometers , and a beam waist diameter of 0.5 mm.

laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm2, an optimal energy density of 9–10 J/cm2, and a corresponding specific ablation energy of 25–30 J/mm3 are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency.

The purpose of this article is to briefly describe the main physical mechanisms of laser ablation of biological tissues and to analyse the prospects of new laser systems for transmyocardial laser revascularisation (TMLR).

to the CHClF2/He mixture irradiated by a Q-switched CO2 laser leads to oxidation of the dissociation product according to the reaction: CF2+NO2→COF2+NO. The resulting COF2 with a 13C content near 50% is easy to convert to CO2 or CO for further enrichment by a nonlaser process. We measured the dependence of the fraction of dimerised CF2 on NO2 pressure pNO2 and the amount of NO2 required to suppress dimerisation on the dissociation yield. Both agree with a kinetic model using known rate constants. For the range of the dissociation parameters (13CF2 yield of 10% per pulse, isotope selectivity of 130) of practical interest, 95% of the CF2 produced is oxidized at pNO2≈1/2pCHClF2. In the absence of NO2, major (20%–35%) losses of CF2 at the metal walls of the irradiation system were observed. Addition of NO2 suppresses them. For comparison, we also used O2 as a scavenger in CHClF2 dissociation. NO2 is by orders of magnitude more efficient.

The 13C-selective IR multiphoton dissociation of CF2CICH3via the vibrational excitation of the C–C bond produces 13CF2CH2 and CF213CH2; high 13C selectivity and production yield can be attained at low laser fluence.

The small absorption is a major problem in isotopically selective IR multiphoton dissociation. Usually the radiation diverges before it is fully absorbed. To solve this problem, we have demonstrated the application of a refocusing (Herriott) multipass cell. It can generally help in laser isotope separation to use the photons more efficiently. Employing such a cell and a Q-switched CO2 laser at high repetition rate, the dissociation yield of CHCIF2 was 23 times higher than in a single pass. The number of passes used (up to 60) was more than is conventionally possible in such small cells. The increased number was permitted by making use of spherical aberration. With 18 passes, we also measured the multiphoton absorption for various wavelengths and pressures, in part separately for12CHCIF2 and13CHCIF2, and also for two-wavelength irradiation. Appropriate change of pressure or wavelength increased the absorption. But the corresponding increase of the dissociation was larger in every case. To explain this and other observations, we invoke the molecular distribution over the energy levels.

In the present work~ the limiting energy characteristics of a 4.3-~m pulsed COa laser are determined, together with the conditions necessary for their realization. The influence of various physical processes occurring in the active medium on the output parameters of the laser system is analyzed. To this end, the generating process in a COa:Na:He mixture preliminarily excited in an electric discharge is subjected to numerical analysis. Model of the Active Medium A diagram of some of the lower vibrational levels of the CO= molecule is shown in Fig. i, together with possible channels of dumping and generation in a 4.3-pm laser. It is quite obvious that in dumping and generation the quasiequilibrium distribution of molecules over levels of antisymmetric type of vibration is disrupted. Therefore, in the analysis of the system, it is necessary to use the level diagram and take account of the fastest relaxation processes involving states with antisymmetric-vibration quanta: intramode vibrational--vib rational (V--V)exchange kvv CO~ t

The study performed indicates the important role of the pulling effect of generation frequencies in molecular lasers with resonance optical excitation and is common for pumping transitions and generation by lasing levels. The magnitude of pulling Δω in a model system (NH3 laser) must reach hundreds of megahertz. A feature of the pulse regime is the Se dependence of Δω and the possibility of change of direction of pulling during generation. Besides it was discovered that in similar systems one can control the temporal parameters of the output pulse by changing the tuning of the highly selective resonator containing a Fabry-Perot interferometer.