Nanosecond laser-pulse experiment

Garcia-Araez N, Climent V, Feliu JM. 2008. Evidence of water reorientation on model electrocatalytic surfaces from nanosecond-laser-pulsed experiments. J Am Chem Soc 130 3824-3833. 

V. WATER REORIENTATION ON SINGLE-CRYSTAL ELECTRODES FROM NANOSECOND LASER-PULSED EXPERIMENTS. POTENTIAL OF MAXIMUM ENTROPY OF DOUBLE-LAYER FORMATION... 

Figure 8. Scheme of the experimental setup of the CIS experiment. Two Fourier-transform-limited nanosecond laser pulses with different frequencies are interacting with cold molecules or van der Waals complexes in a skimmed supersonic molecular beam. 

Our experiments with tryptophan (Trp) and a tryptophan-containing tripeptide revealed some specific features that are especially evident from comparison between the mass spectra resulting from irradiation with femtosecond and nanosecond laser pulses in the UV (308 nm) and visible (620 nm) regions of the spectrum. The degree of fragmentation depends on both... 

The experiments described above used nanosecond laser pulses, which are much longer than the rotational period of the molecules. At the termination of the pulse, the pendular state that is formed relaxes adiabatically to a free-rotor eigenstate. If instead picosecond laser pulses are used, a rotational wave packet is formed by successive absorption and re-emission of photons during the laser pulse. Such wave packets are expected to display periodic recurrences of the alignment after the end of the pulse. 

The incident probe pulse was the 532 nm second harmonic beam from a Coherent Infinity Nd YAG laser. The 3 nanosecond laser pulse was oriented 60 degrees from the water surface normal direction. The 6 millimetre diameter laser beam was not focussed, and the beam energy was 85 mJ per pulse. The imaging detection system described in Part 1 was used in this experiment it consisted of a dichroic image splitter, quartz Nikon camera lens, and a pulsed gated, intensified CCD camera (Roper Scientific formerly Princeton Instruments). 

The 2AP and B[o]PT absorption bands are positioned above 300 nm, and are thus beyond the threshold of the absorption spectrum of DNA. Thus, the canonical DNA bases are not photoionized by either 308- or 355-nm nanosecond laser pulses used in these experiments. 

Picosecond laser pulses in the UV range do not result in better ablation behavior than nanosecond laser pulses. This is different for doped polymers. Experiments with doped PMMA (an IR-absorber, i.e., IR-165 for ablation with near-IR laser and diazomeldrum s acid (DMA) for ablation with UV lasers) with nanosecond and picosecond laser irradiation in the UV (266 nm) and near-IR (1064 nm) range have shown that, in the IR, neat features could be produced with picosecond laser irradiation, while nanosecond irradiation only results in rough surface features [105]. This corresponds well with the different behavior of the two absorbers. With IR-165 the polymer is matrix is heated by a fast vibrational relaxation and multiphonon up-pumping [106]. This leads to a higher temperature jump for the picosecond irradiation, which causes ablation, while for nanosecond pulses only lower temperatures are reached. 

In the earlier experiments reported in the work by Khoo and Normandin, nanosecond laser pulses from the second harmonic of a Q-switched Nd YAG laser... 

The last technique employed by these authors is very useful because it allows to do femtosecond time resolved experiments simply by using incoherent nanosecond laser pulses. 

The formation of 2-phenylbenzofuran during the nanosecond laser flash experiment was corroborated by a picosecond study of 8a. A rise time of 2 to 4 ps was determined for the 340 nm transient. A rich fluorescence emission obtained in the nanosecond study was shown to arise from 9 generated during the nanosecond laser excitation pulse. Naphthalene also quenched the formation of 9 at the same rate as the formation of 10, estabhshing that the two primary photoproducts came from the same triplet (i.e., 8a). Thus, for the unsubstituted benzoin phosphates, reaction proceeds exclusively through the triplet manifold... 

In the previous section we discussed light and matter at equilibrium in a two-level quantum system. For the remainder of this section we will be interested in light and matter which are not at equilibrium. In particular, laser light is completely different from the thennal radiation described at the end of the previous section. In the first place, only one, or a small number of states of the field are occupied, in contrast with the Planck distribution of occupation numbers in thennal radiation. Second, the field state can have a precise phase-, in thennal radiation this phase is assumed to be random. If multiple field states are occupied in a laser they can have a precise phase relationship, something which is achieved in lasers by a teclmique called mode-locking Multiple frequencies with a precise phase relation give rise to laser pulses in time. Nanosecond experiments... 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

The delay time between the pump and the probe laser pulses is usually very short in these experiments. The delay time is less than 5 ns when the pump and the probe laser pulses are the same, and the delay time is as long as several hundred nanoseconds when the pump and the probe laser pulses are from two different sources. The short delay time ensures that the fragments flying with different velocities are equally sampled before they leave the detection region. Since the delay time is much shorter than the lifetime of the excited molecules (.A ), most of these molecules do not dissociate into fragments when the probe laser pulse arrives. As a result, the probe laser can easily cause dissociative ionization of the vibrationally excited molecules due to their large internal energy. 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

A survey of the application of high-power pulsed lasers to experiments on a nanosecond time scale has been given by Bradley 2 ). 

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