Robert D. McAlpine

The multiphoton absorption and decomposition of fluoroform‐d: Laser isotope separation of deuterium

Multiphoton absorption (MPA) studies of fluoroform‐d, a molecule of interest for potential laser based hydrogen isotope separation processes, are reported for CDF3 pressures 0.2–1.3 kPa, and for a variety of 10 μm CO2 laser lines with pulse widths of 2 or 6 ns and fluences within the range 10−3–70 J/cm2. Unlike SF6, no red shift of the MPA spectrum relative to the small signal spectrum was observed at high fluence. Selective multiphoton decomposition (MPD) experiments using the 10R(26) line, 6 ns pulse to excite the CDF3 component in natural‐abundance CHF3 (∼ 150 ppm D/H) at a pressure of 13.3 kPa resulted in the recovery of water enriched up to 30% in deuterium—a measured isotope enrichment of ⩾2000 fold. This demonstrates that a product, highly enriched in deuterium, can be recovered from the selective MPD of fluoroform.

Laser isotope separation and the multiphoton decomposition of methanol using a pulsed HF or DF laser

Abstract The focussed beam from a single line [P 1 (6)] pulsed HF laser has been used to decompose CH 3 OH (for pressures between 0.169 and 14.95 kPa) and isotopic mixtures of methanol. The normalized yields (number of product molecules Pulse/ P METHANOL ) of the non-condensable products H 2 , CO and CH 4 increased linearly with pressure (for the range ≈ 1 - 7 kPa). For sufficiently low pressures, selective excitation of one component of an isotopic mixture gives an isotopically enriched product. For example, selective excitation of CH 3 OH in equimolar of mixture CH 3 OH/CH 3 OD at a total pressure of 269 Pa gives hydrogen which is enriched 60-fold in H versus D. The degree of isotopic enrichment decreases with increasing mixture pressure. The efficiency of conversion of photon energy to reaction product has been observed to increase linearly with pressure. Decomposition studies have been performed in the presence of additives. These imply that the decomposition of methanol to H 2 involves mainly molecular rather than free radical steps.

Short-pulse CO(2) laser for photochemical studies.

A line-tunable CO(2) laser system is described producing repeatable pulses with selectable widths from 2 to 60 nsec. High energy contrast ratio (minimum of 10) is achieved in both the P and R branches of the 9- and 10-microm bands making this system a valuable source for photochemical studies.

Multiphoton absorption of HF laser photons by molecules containing a hydroxyl group

Multiphoton absorption (MPA) of HF laser radiation has been studied, as a function of pressure (15 Pa to 1.3 kPa) and fluence (2 mJ/cm2 to 75 J/cm2) for the series: water, methanol, methan‐d 3‐ol, ethanol, and 2,2,2‐trifluoroethanol. As the group attached to the –OH is made more complex, the quasicontinuum occurs after fewer excitation steps, and under ’’collisionless’’ conditions, the same degree of multiphoton excitation is found to require a lower fluence. For water, at pressures between 73 Pa and 1.3 kPa, the cross sections are considerably lower than those for the other molecules, and MPA requires fluences in excess of ∼75 J/cm2. The remaining molecules divide into two groups, the ’’small’’ molecules (CH3OH and CD3OH) and the ’’large’’ molecules (C2H5OH and CF3CH2OH). For the small molecules at low pressures, the cross sections decrease with increasing fluence, an effect which is thought to be due to anharmonic bottlenecking. As pressure increases, the fluence dependence of the cross sections disappear. For the large molecules, anharmonic bottlenecking appears to be reduced, due to the greater density of states, and cross sections increase with increasing fluence according to the empirical form: σ(E, P)=K′E b′ P a (where P is pressure, E is fluence and b′, a, and K′ are constants). The facility of HF laser‐induced collisionless multiphotondissociation of the –OH containing molecules is discussed in light of these results.

Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser

Abstract The focussed beam from a single line [P 2 (5)] of a pulsed HF laser has been used to stimulate the decomposition of formic acid. The yield ( Y is the number of product molecules per pulse / formic acid pressure) of the non-condensable (77 K) products, hydrogen and CO, has been studied as a function of laser radiant energy (from 25-115 mJ) and pressure (from 0.4-2.7 kPa). The intensity dependence of Y suggests that each dissociating formic acid requires the equivalent of at least 6 HF P 2 (5) photons (260 kJ/mole). For pressures above about 0.6 kPa, Y H 2 = (−0.6 ± 1.7) × 10 12 + (2.4 ± 1.0) × 10 12 P and Y CO = (−0.5 ± 6.1) × 10 13 × (8.7 ± 3.7) × 10 13 P. The linear dependerrce of yields indicates that a collisionally assisted decomposition process is important at these pressures. The efficiency of the conversion of photon energy to reaction products at a pressure of 2.7 kPa is ⩾ 7% for CO and ⩾ 0.2% for hydrogen. Selective excitation of HCOOH in equimolar mixtures of HCOOH/HCOOD, at a total pressure of 0.6 kPa, has provided a physically separated product, hydrogen gas, which is isotopically enriched in H versus D 25 fold as compared to the formic acid mixture. The degree of enrichment decreases as the total pressure of the mixture is increased. A possible mechanism accounting for isotope enrichment and the collisionally assisted dissociation is outlined.

CO2 laser‐induced multiphoton absorption of fluoroform‐d: The effects of collisionsa)

Studies of the multiphoton absorption of the 10 μm R(26) CO2 laser line by fluoroform‐d have been extended to laser pulse widths (Δτ) of 60 ns over the range of pressures P=0.33 to 2.00 kPa. Comparison with earlier studies with Δτ=2 and 6 ns shows that for all fluences Φ the cross section σ=σ(Φ, PΔτ), which means that σ scales with the number of collisions during the pulse. Log–log plots of σ(Φ) vs Φ (P, Δτ fixed) show a bend at a fluence Φc. For Φ<Φc collisions appear mainly to induce rotational relaxation to fill the hole ‘‘burned’’ by the laser in the ground state rotational distribution. For Φ≳Φc the role of collisions is more complex and likely involves intramolecular relaxation processes.

Multiphoton absorption of intense HF laser radiation by methanol

Abstract Cross sections for absorption of the HF P 1 (6) line by methanol were obtained for pressures between 4.2 Pa and 2.75 kPa. For pressures below 200 Pa, cross sections decrease with increasing laser fluence. However for higher pressures cross sections are independent of fluence. Collisional perturbations are likely responsible for this change of behaviour.

Pulsed CO2 laser induced multiphoton decomposition of cyclobutanone: A comparison of predictions for three models of decomposition probability vs. fluence

The multiphoton decomposition (MPD) probability (f) for photolysis of cyclobutanone (pressure: 0.333 kPa) with a pulsed (pulse width: 40 ns) CO2 laser tuned to the 9P(20) line, was studied,in a collimated beam geometry, as a function of fluence (ϕ=0.75 to 11.7 J/cm2). The values off (ϕ) were fitted to three suggested 1- or 2-parameter models. Models with two parameters correlated the experimental values equally well, whereas a 1-parameter model was less accurate for low fluences. The MPD yield for two focussed beam geometries was measured and compared to predicted yields obtained by integrating the three models forf (ϕ) over the focussed beam geometries. All three models predicted yields which agreed well with the measured values.

Laser isotope separation and the multiphoton decompotions of formaldehyde using a focused DF laser: the effect of single- or multi-line irradiation

Abstract Formadehyde was decomposed by a focused, pulsed DF laser. Simultaneous irradiation with multiple DF lines gives yields almost 100 times those obtained with single DF line irradiations of comparable total energy. Selective irradiation of H2CO in equimolar li2CO/D2CO gives large hydrogen isotope enrichment for low pressures.

Intensity dependence of CO2-laser-induced multiphoton absorption and decomposition of monomethylamine

Abstract The infrared multiphoton absorption (MPA) and decomposition (MPD) of monomethylamine have been studied using temporally smooth CO 2 -laser pulses of 6,9 and 40 ns fwhm. Both MPA and MPD depend on gas pressure (0.267–1.33 kPa) at each pulse width; but results for different pulse widths did not simply scale with collision number. At each pressure, MPA and MPD depend on laser-pulse intensity as well as fluence confirming the suggestion of previous workers.