Progress in circular dichroism laser mass spectrometry

Studies on laser ablation of silicon using near IR picosecond and deep UV nanosecond lasers

In mass spectrometry enantiomers can be distinguished by multiphoton ionization employing circular polarized laser pulses. The circular dichroism (CD) is detected from the normalized difference in the ion yield after excitation with light of opposite handedness. While there are cases in which fragment and parent ions exhibit the same sign of the CD in the ion yield, several experiments show that they might also differ in sign and magnitude. Supported by experimental observations it has been proposed that the parent ion, once it has been formed, is further excited by the laser, which may result in a change of the CD in the ion yield of the formed fragments compared to the parent ion. To gain a deeper insight in possible excitation pathways we calculated and compared the electronic CD absorption spectra of neutral and cationic (R)-3-methylcyclopentanone, applying density functional theory. In addition, electron wavepacket dynamics were used to compare the CD of one- and two-photon transitions. Our results support the proposed subsequent excitation of the parent ion as a possible origin of the difference of the CD in the ion yield between parent ion and fragments.

The feasibility of measuring circular dichroism (CD) in ion yields is demonstrated in multiphoton ionization employing femtosecond laser radiation for the first time. CD values for (R)-propylene oxide are reported at 738 nm, 810 nm and 878 nm. The data suggest the possibility of resonance contributions at 810 nm. At the other wavelengths the CD appears to originate from non-resonant multiphoton ionization. Possible implications of measuring non-resonant CD are discussed.

The 1Σ+(0+) and 3Σ−(0+) Rydberg states as well as the V 1Σ+(0+) valence state of hydrogen chloride isotopomers are studied using (2+1) resonance enhanced multiphoton ionization (REMPI) and time-of-flight mass spectrometry (TOFMS). Pulsed supersonic expansion provides sufficient rotational cooling to facilitate rotational analysis. Polarization selection in conjunction with fragment and isotope discrimination provide the first unambiguous identification of all vibrational bands from v′=3 to 32 of the V 1Σ+(0+) state. REMPI excitation spectra from the v″=0 level of the X 1Σ+(0+) state through vibrational levels of the broad V 1Σ+(0+) state suggest non-Franck–Condon transitions. Perturbations in intensity, line position, and rotational constants are consistent with the near-adiabatic nature of the short-range E 1Σ+(0+) Rydberg state and long-range V 1Σ+(0+)state. Evidence for Rydberg∼valence interaction and repulsive superexcited states is provided by the detection of strong H+ and Cl+ ion yields. The onset of a distinct Cl+ channel is detected for the first time and confirms the presence of at least two dissociation pathways at the three-photon energy and at large internuclear distance.

Changes in surface morphology and ablation rate induced on sapphire were investigated after interaction with femtosecond laser pulses in air at variable fluence (2 to 77 J cm−2) and repetition rate (10 to 1000 Hz). Multiple laser pulses at a wavelength of 785 nm and pulse width of 130 fs were fired at the surface of sapphire to produce craters whose depth, size and morphology were evaluated using optical and scanning electron microscopy. Ablation rate was found to depend on laser fluence, number of laser pulses and repetition rate. A rapid increase in ablation rate with fluence was observed for fluences lower than 5.9 J cm−2, followed by a slow increase up to fluence of 40.7 J cm−2. A drop in ablation rate occurred at fluence greater than 40.7 J cm−2. Craters produced at high repetition rate (1000 Hz) at fluence of 11.8 J cm−2 were deeper than those produced at low repetition rate (10 Hz) during the first 40 to 50 pulses. The situation was reversed for craters produced by greater than 50 laser pulses. The drop in ablation rate observed at high fluence and repetition rate can be attributed to attenuation of the laser energy due to plasma and particle shielding that result from interactions with the laser-generated particles that cannot be completely removed from the ablated crater. Defocusing effects associated with the non-equilibrium ionization of air which causes a divergence to the laser beam and consequently a reduction in the laser intensity at the sample surface can be another reason for the observed drop in the ablation rate at high fluence.

The combination of circular dichroism with laser mass spectrometry via the measurement of ion yields is a powerful tool in chiral recognition, but the measured anisotropies are generally weak. The method presented in this contribution reduces the measurement error significantly. A common path optical setup generates a pair of counter-rotating laser foci in the interaction region of a time-of-flight spectrometer. As the space focus condition is fulfilled for both foci individually, this becomes a twin-peak ion source with well separated and sufficiently resolved mass peaks. The individual control of polarization allows for in situ correction of experimental fluctuations measuring circular dichroism. Our robust optical setup produces reliable and reproducible results and is applicable for dispersion sensitive femtosecond laser pulses. In this contribution, we use 3-methyl-cyclopentanone as a prototype molecule to illustrate the evaluation procedure and the measurement principle.

In this experimental and theoretical work we focus on the technique of pulsed laser annealing applied to the metastable ferromagnetic semiconductor GaMnAs. Analytical heat-flow calculations are used to illustrate the position and time-dependent temperature distribution during the whole laser annealing process. Such heat-flow calculations will also play an indispensable role for the preparation of other new metastable diluted ferromagnetic semiconductors by ion implantation and subsequent laser annealing. The structural, magnetic, and magnetotransport properties of ferromagnetic GaMnAs have been probed in dependence on the annealing parameters, e.g., the number of laser pulses and the pulse length. Annealing with a single KrF laser pulse of 30 ns and $0.26\text{ }\text{J}/{\text{cm}}^{2}$ with the photon energy above the GaAs band gap energy leads to similar magnetic properties like annealing with a single 3 ns Nd:YAG laser pulse with the photon energy below the GaAs band gap energy. We observed that possibly due to Mn diffusion and decreasing hole concentration, several laser pulses degrade the structural and magnetic properties of GaMnAs. Our results reveal the largest saturation magnetization in Mn-implanted GaAs annealed with a single KrF laser pulse.

Large area, high speed, nanopatterning of surfaces by laser ablation is challenging due to the required high accuracy of the optical and mechanical systems fulfilling the precision of nanopatterning process. Utilization of self-organization approaches can provide an alternative decoupling spot precision and field of machining. The laser-induced front side etching (LIFE) and laser-induced back side dry etching (LIBDE) of fused silica were studied using single and double flash nanosecond laser pulses with a wavelength of 532 nm where the time delay ∆τ of the double flash laser pulses was adjusted from ∼50 ns to ∼10 μs. The fused silica can be etched at both processes assisted by a 10 nm chromium layer where the etching depth ∆z at single flash laser pulses is linear to the laser fluence and independent on the number of laser pulses, from 2 to 12 J/cm2, it is ∆z = δLIFE/LIBDE ⋅ Φ with δLIFE ∼ 16 nm/(J/cm2) and δLIBDE ∼ 5.2 nm/(J/cm2) ∼ 3 ⋅ δLIFE. At double flash laser pulses, the ∆z is dependent on the time delay ∆τ of the laser pulses and the ∆z slightly increased at decreasing ∆τ. Furthermore, the surface nanostructuring of fused silica using IPSM-LIFE (LIFE using in-situ pre-structured metal layer) method with a single double flash laser pulse was tested. The first pulse of the double flash results in a melting of the metal layer. The surface tension of the liquid metal layer tends in a droplet formation process and dewetting process, respectively. If the liquid phase life time ∆tLF is smaller than the droplet formation time the metal can be frozen in an intermediated state like metal bare structures. The second laser treatment results in a evaporation of the metal and in a partial evaporation and melting of the fused silica surface, where the resultant structures in the fused silica surface are dependent on the lateral geometry of the pre-structured metal layer. A successful IPSM-LIFE structuring could be achieved assisted by a 20 nm molybdenum layer at ∆τ ≥ 174 ns. That path the way for the high speed ultra-fast nanostructuring of dielectric surfaces by self-organizing processes. The different surface structures were analyzed by scanning electron microscopy (SEM) and white light interferometry (WLI).

The circular dichroism manifested in the photoionization yields of the chiral molecule fenchone has been investigated using two laser beams of different wavelengths intersecting at 90 degrees. This allowed different two-colour ionization schemes to be investigated so that the circular dichroism contributions of each step in the multiphoton ionization process could be deduced. The results showed that the circular dichroism generated by a beam at a wavelength of 260 nm could be strongly influenced by the polarization state of a 520 nm beam propagating in a perpendicular direction. The asymmetry in the ion yields was found to change depending on whether the 520 nm beam was left or right circularly polarized, and if it was linear polarized in a direction parallel or perpendicular to the propagation direction of the 260 nm beam. The control exhibited by the direction of the linearly polarized light is attributed to an orientation dependence of the circular dichroism due to selective excitation of the isotropic ensemble by the 520 nm beam. By contrast, when both beams were circularly polarized, the dependence on the polarization direction of the 520 nm pulse is ascribed to either the interference between different ionization pathways or excitation of a chiral vibronic wavepacket in the molecule. These results are the first demonstration of chiroptical quantum control of total ion yields, showing in principle that all-optical enantio-sensitive chemical processing may be possible in the future.

Parametric study on femtosecond laser pulse ablation of Au films

We report two new optical detection methods for the CH<SUB>2</SUB> (X <SUP>3</SUP>B<SUB>1</SUB>) radical and one new detection method for the HCF (X <SUP>1</SUP>A') radical. These detection methods are based upon resonance enhanced multiphoton ionization (REMPI) spectroscopy and require only commonly available lasers. Single laser pulse sensitivity for CH<SUB>2</SUB> and HCF is better than 10<SUP>8</SUP> radicals(DOT)cm<SUP>-3</SUP>. REMPI spectra of CH<SUB>2</SUB> between 380 and 440 nm arose from three-photon resonances with the B <SUP>3</SUP>A<SUB>2</SUB> (3d), C (3d), D (3d), and 4d <SUP>3</SUP>A<SUB>2</SUB> Rydberg states between 78,950 and 68,200 cm<SUP>-1</SUP> above the ground state. A fourth laser photon ionized the radicals and the signal was carried by CH<SUB>2</SUB><SUP>+</SUP> (m/z 14) ions, i.e., CH<SUB>2</SUB> signal arises through a 3 + 1 REMPI mechanism. We have also discovered two new states, H(3p) and I(4p), that enable extremely sensitive detection of CH<SUB>2</SUB> through a 2 + 1 REMPI scheme. In two photons, the H(3p) state produces a strong band at 311.80 nm (64,126 cm<SUP>-1</SUP>). The much weaker I(4p) state appears at 269.27 nm (74,254 cm<SUP>-1</SUP>). Between 305 and 325 nm HCF and DCF radicals produce 2 + 1 REMPI spectra by excitation to 3p Rydberg states. The band origins for HCF and DCF are at 321.6 nm (62,180 cm<SUP>-1</SUP>) and 321.7 nm, respectively.

Surface ablation of PLLA induced by KrF excimer laser

A newly developed compact and mobile time-of-flight mass spectrometer (TOFMS) for on-line analysis and monitoring of complex gas mixtures is presented. The instrument is designed for a (quasi-)simultaneous application of three ionization techniques that exhibit different ionization selectivities. The highly selective resonance-enhanced multiphoton ionization (REMPI) technique, using 266-nm UV laser pulses, is applied for selective and fragmentationless ionization of aromatic compounds at trace levels (parts-per-billion volume range). Mass spectra obtained using this technique show the chemical signature solely of monocyclic (benzene, phenols, etc.) and polycyclic (naphthalene, phenathrene, indol, etc.) aromatic species. Furthermore, the less selective but still fragmentationless single photon ionization (SPI) technique with 118-nm VUV laser pulses allows the ionization of compounds with an ionization potential below 10.5 eV. Mass spectra obtained using this technique show the profile of most organic compounds (aliphatic and aromatic species, like nonane, acetaldehyde, or pyrrol) and some inorganic compounds (e.g., ammonia, nitrogen monoxide). Finally, the nonselective ionization technique laser-induced electron-impact ionization (LEI) is applied. However, the sensitivity of the LEI technique is adjusted to be fairly low. Thus, the LEI signal in the mass spectra gives information on the inorganic bulk constituents of the sample (i.e., compounds such as water, oxygen, nitrogen, and carbon dioxide). Because the three ionization methods (REMPI, SPI, LEI) exhibit largely different ionization selectivities, the isolated application of each method alone solely provides specific mass spectrometric information about the sample composition. Special techniques have been developed and applied which allow the quasi-parallel use of all three ionization techniques for on-line monitoring purposes. Thus, a comprehensive characterization of complex samples is feasible jointly using the characteristic advantages of the three ionization techniques. Laboratory applications show results on rapid overview characterization of mineral oil-based fuels and coffee headspace. The first reported field applications include timely resolved on-line monitoring results on automobile exhausts and of waste incineration flue gas.

It has been shown by numerical simulation that the dissipative soliton resonance suppresses the appearance of new ultrashort pulses in the laser cavity that usually arise with increasing pumping. As a consequence, the energy of the pulses can reach an arbitrarily large value, which is determined by the corresponding pump power. The mechanism of the suppression is due to interaction between pulses through a saturable gain medium. The generation remains multistable: the number of pulses in steady-state operation depends on the initial conditions. In the case of multipulse generation, all pulses have identical durations, shapes, peak intensities, and chirps. The effect of maintaining single pulse operation with increasing pump power can be used to generate high-energy pulses.

Effects of laser parameters on morphological change and surface properties of aluminum alloy in masked laser surface texturing