W. Fuss

A Sensitive Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

Abstract We present a nondispersive infrared spectrometer (NDIRS) for the measurement of the 13CO2/12CO2-ratio in breath samples. A commercial NDIR spectrometer for CO2 concentration measurements in industrial process control was modified using two separate optical channels for the 13CO2 and 12CO2 detection. Cross interference due to overlapping absorption lines of both isotopic gases was successfully eliminated. The sensitivity of this device is ± 0.4‰ of the 13CO2/12CO2-ratio in a range of 2.5 to 5% of total CO2. This is sufficient for biomedical applications. Our spectrometer is small in size, cheap and simple to operate and thus a true alternative to isotope ratio mass spectrometers (IRMS). Several biomedical applications with breath samples were demonstrated and were compared in very good agreement with IRMS.

Macroscopic Isotope Separation of 13C by a CO2 Laser

Abstract By photochemical dissociation of the rare carbon isotope component of CHClF2 by means of a CO2 laser with an average power of 150 W, Q-switched at 10 kHz, we have demonstrated the separation of more than 1 mol of 13C, enriched to 50% (2 mol of total carbon). It is contained in about 1 mol (101 g) of the product C2F4. The total throughput of the starting material was 29 kg. The experiment was run day and night for 2 weeks, almost only controlled by a computer. We obtained production rates of 5 mmol/h, corresponding to about 0.5 kg 13C per year.

Multiphoton dissociation in a focused Gaussian beam

If a Gaussian beam is focused into the center of a cell, the conversion U can be calculated from known or assumed dissociation probabilities Pd(Φ) (Φ = energy density). Conversely, for several practically important model functions for Pd that depend on one or two parameters, this Pd can be recovered from U, within limits even for non-Gaussian beams. The same technique has been applied to an ionization probability function often used in multiphoton ionization.

13C-separation by a continuous discharge CO2 laser q-switched at 10 kHz

Abstract Compared to atmospheric pressure pulsed CO 2 (TEA) lasers, continuous-discharge CO 2 lasers can deliver photons at much lower costs. By a Q-switched version of such a laser we demonstrated by multiphoton dissociation of CHClF 2 the production of 1 g of C 2 F 4 enriched to 50% 13 C. This is a larger quantity of high enrichment than has been produced so far using TEA lasers. The process is already automated to a large extent. Scaling up seems feasible. These results have been achieved, although the process turned out to be much more nonlinear for our pulses (length 200 ns) than for TEA laser pulses. This difference is attributed to intensity effects, caused by direct multiphoton transitions in the excitation ladder. To avoid that selectivity breaks down, the gas has to be exchanged more than once per pulse. The required speed is much less, if ~250 mbar of Ar are added to the working gas.

Advances in the development of liquid-core waveguides for IR applications

With the development of infrared transmitting fibers, medical applications such as minimally invasive surgery are becoming feasible. In particular we investigate liquid core waveguides with an Er:YAG laser at 2.94 micrometer. Because of their advantages like variability in diameter, high flexibility, and mechanical stability, liquid core waveguides appear to be an alternative to conventional IR waveguides. In this work we present two types of liquid CCl4 filled lightguides that have been developed with plastic tube and quartz capillary as cladding. The former with an inner diameter of 1.6 mm showed an attenuation of 2.6 dB/m at 2.94 micrometer. For the quartz glass capillary with an inner diameter of 550 micrometers an attenuation of approximately 4.8 dB/m was determined in first experimental results. Due to the great flexibility and the high mechanical stability of both lightguides, bending radii below 10 mm are possible. Transmission losses depending on bending radii are discussed. A comparison between measurements with an IR-spectrometer and an Er:YAG laser shows that a minimum transmission loss of 2 dB/m can be achieved.

New Developments In High-Power CW Discharge Multikilohertz Repetition Rate CO 2 Lasers

Based on a continuous discharge slow gasflow CO. laser a high peak power, medium average power, high repetition rate laser has been developed. The system consists of a Q-switched oscillator plus a subsequent laser amplifier. Q-switching is done by a fast spinning chopper disk placed in the focal plane of an intracavity cylindrical telescope. Almost 500 W of average laser power have been achieved in the pulsed mode at 20 kHz repetition rate from laser tubes rated for 1 kW power in cw oscillator mode. Single pulse energies are 27 mJ to 50 mJ ( depending on duration of the tail) at repetition rates up to 10 kHz on line 9P(20). Up to to 50 kHz are easily achievable at reduced single pulse energy. Pulse duration is 200 ns (FWHM). The peak power of 150 kW is more than one order of magnitude higher than that with discharge current switching techniques in low pressure lasers. Therefore lasers of this type are suitable for applications in nonlinear processes like IR photochemistry or material processing without sacrificing much of the advantages of cw lasers as compared to TEA lasers. Investigations on stored energy, dependence on repetition rate, self oscillation problems, switching speed requirements and scaling properties are also reported.

Probing of ultrafast photoinduced isomerization and dissociation reactions by intense-field dissociative ionization

Ionization by the electric field of intense laser radiation offers some advantages to study the dynamics of ultrafast chemical reactions in pump-probe experiments: any electronically excited state is expected to give an enhanced ion signal independent of its electronical and vibrational nature, depending mainly on its ionization potential; ionization is followed by extensive fragmentation which providers additional information on transient intermediate states and/or species. We applied this approach to study the femtosecond dynamics of a number of photoinduced isomerization and dissociation reactions in the gas phase including electrocyclic ring opening of 1,3-cyclohexadiene, pericyclic hydrogen migration of 1,3,5-cycloheptatriene, UV photodissociation of metal hexacarbonyls. Monitoring the pump-probe delay kinetics of the parent and various fragment ions, we are able to follow the molecule from state to state all along the primary photochemical reaction path which is completed within 200 fs. The common feature of these different chemical reactions is the ultrafast passage from S1 to S0 within a time in the range of 40 to 80 fs. Such ultrafast processes can only take place, if there is a continuous pathway between the two potential surfaces. The observations are therefore a strong support for the S1/S0 conical intersections predicted by quantum chemistry.

PULSED INFRARED SELECTIVE DISSOCIATION OF CF3BR AS A FUNCTION OF PRESSURE,TEMPERATURE, FLUENCE AND WAVELENGTH

The pulsed InfraRed (IR) photodissociation of pure CF3Br and a 1:3 mixture with H2 has been studied as a function of pressure, temperature, fluence and wavelength. The results have been obtained by mass spectrometry. Maximum enrichment for carbon 13 is obtained when irradiating with the 1046.9 cm−1 (9P20) line, but the selectivity is stronger at longer wavelengths. The enrichment factor tends towards 1 when pressure increases with the pure substance, but it is less affected in the mixture. It seems that hydrogen has a buffer effect. The selectivity shows maxima for all lines with the fluence and increases when the temperature is decreased from 22 to − 85°C. Some enrichment is observed for the bromine 79 isotope when irradiating with 1046.9 cm−1 (9P20).

Scaling of high-repetition-rate Q-switched CO2 lasers for industrial applications

By insertion of a fast mechanical Q-switch into the resonator, continuous discharge CO2 lasers can yield high peak power pulses at multi-kHz repetition rate. First experiments have been done with diffusion-cooled low-pressure CO2 lasers. For this type of laser the pulse energy can only be increased by increasing the length of the active medium. A limit is given by the onset of uncontrolled self oscillation which prevents regular Q-switched operation. Single pulse energies can apparently not exceed 30 mJ at 250 ns pulse duration for this type of laser. Fast gas-flow convection-cooled laser discharges allow us to increase the stored energy by increasing diameter and pressure of the active medium as well as the electrical power density. We present the results of Q-switching of a 5 kW industrial laser. Our Q-switch is scalable in optical power. It is based on a fast chopper and a conical mirror. In some experiments we tuned the laser over a wide range by a diffraction grating. The influence of gas pressure and mixtures as well as discharge parameters has been studied. Single pulse energies of 100 mJ have been found, limited by the electrical input power density.

High-Power cw Discharge Multikilohertz Repetition Rate Q-Switched CO2 Lasers

In order to demonstrate the usefulness of CO2 lasers in laser chemistry, we intend to use the isotope selective multiphoton dissociation of CHClF2 to separate carbon 13 with a natural abundance of 1% from carbon 12 with an abundance of 99%. This process has first been demonstrated by Hackett and coworkers /1/, using a TEA –CO2 laser. In a previous paper /2/ we have predicted that this process would take place efficiently at pulse energies down to 50 mJ if the pulse duration is in the order of several hundred nanoseconds. By using a multipass reaction cell the photons are fully absorbed in spite of the low gas density required to maintain the isotope selectivity. The pulse repetition rate should be high to provide a high conversion rate. The rate is only limited by the time required to replace the molecules in the irradiated gas volume between subsequent pulses. As the multiphoton dissociation works only at energy densities above several Joules per cm2 the laser beam has to be refocussed many times in the multipass cell requiring good beam quality. In short the following laser parameters are desirable to make the isotope separation process work and to yield several hundred grams of carbon 13 at a rate of about 10 g per day.