Laser light source pdf

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Remove Product. Remove this product from your comparison list? Check Order Status. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.

The reason for this is sent a unique and powerful probe of ultrafast electronic, simple—strong-field ionization has few selection rules, and atomic, and molecular dynamics. Any excited transition-state ions that might be generation7,8 and carrier-envelope-phase stabilization9—11 created during the process are rapidly ionized themselves.

In monic sources. This topic ics. These dynamics are directly relevant to naturally occur- is of interest for understanding radiation chemistry at the ring processes such as solar radiation-induced processes in most-fundamental level. Nevertheless, experiments in this planetary atmospheres. Generally, in a pump-probe study, a significant excited states of molecules that are not directly accessible fraction of a target sample must be excited by using a pump using multiphoton techniques.

In the past, different schemes have been used to generate EUV light for spectroscopic purposes, e. These sources have limited time resolution, and are thus incapable of observing these processes in real time. Since real-time observation can also facilitates the potential for real-time control, there is a strong incentive to develop ultrashort-pulse EUV sources.

Recently developed vuv and x-ray free electron lasers; i. The EUV is then which is more than sufficient to capture ultrafast molecular filtered and focused to the interaction region by using multilayer EUV mir- dynamics, and even directly observe purely electronic rors. The IR is time delayed using an adjustable beam path length. In this paper, we demonstrate that it is possible to effec- tively make use of these laser-based EUV sources for direct studies of molecular dynamics induced by ionizing radiation.

Second, we need to A schematic of the experimental setup is shown in Fig. In this article, we discuss improvements in the HHG. The EUV and IR beam delivery optics, and the implementation of efficient beams are then spatially overlapped on a molecular gas jet, spectroscopic and imaging scheme for the experiment.

By be discussed in greater details in the following text. Laser system dynamics of highly excited states in small molecules. In this doped sapphire-based ultrafast laser oscillator-amplifier article, we discuss the techniques and methods which we system patterned after the KMLabs Inc.

Ti:sapphire-based laser amplifier34,35 with a ring geometry using a cryogenically cooled laser amplifier crystal to avoid thermal lens distortions. The laser generates pulses of 2 mJ II. Most experiments were nents: the laser system and high harmonic upconversion performed at 2 kHz.

The laser system employs a standard setup, and the momentum imaging spectrometer. The laser grating-based stretcher and compressor, which makes pos- system will be discussed first, followed by a summary of the sible excellent amplifier spatial mode quality at high power. More in- The output beam has a well-behaved focus with an M 2 of depth descriptions of the HHG process have appeared in past 1. Downloaded 12 Jun to In this work, we use a 2.

In Eq. The inner capillary is left minimally deformed by the hole. The hourglass shape seen in the picture is first term within the parentheses corresponds to the neutral due to the lensing effect of the melted outer wall of the capillary. The middle section can be changed to allow the curly brackets takes the waveguide contribution to the different length waveguides to be used.

Thus, the EUV signal can gation inside a hollow core waveguide and found that even build up coherently along the propagation direction. The Ar gas pressure inside launched. The modular design pulse intensity and duration, and which harmonics are to be makes it possible to insert various length V-groove center optimized.

The waveguides are made of fused silica and are sections. To prevent The quality of the output spatial mode of the HHG disruption of the propagating IR laser mode due to uneven waveguide is important. The result of such a laser-created hole can be detector. For the EUV beam, this can only be achieved if the seen in Fig. HHG output spatial mode is well behaved. Because the optimal phase guide and the degree of phase matching. The blue curve is a broadband reflection. The red curve is narrow-band reflection.

For many molecular dy- throughput, and thus the flux on target. However, these are not the optimum ma- adequate. Increasing the reflectiv- pulses, very short driving laser pulses can be used. Such bandwidth selection can be considerably less loss. However, wavelength selection using a grating will result in broadening the C.

Pump-probe beam path pulse envelope in the time domain, with a corresponding For the molecular dynamics experiments, we used two reduction in temporal resolution.

The beam path geometry monochromator. Multilayer mirrors, on the other hand, Sec. The beam path geometry shown in Fig. The dis- where interferometric mechanical stability is required.

The reflection spectrum of these mirrors can be tuned from narrow band to broadband, depending on choice of coating parameters. The dashed line corresponds to the IR beam. These sideband the EUV beams together.

The IR pulse beam path is adjustable in The multilayer pair used in these experiments has a length to vary the time delay between the EUV and IR pulses. However, further improve- stable measurements. A cross correlation in a frequency-doubling crystal is used to find the temporal overlap. To achieve spatial overlap, a lens placed past the detector chamber intersects the EUV and IR paths, and a pinhole is placed at the point that images the interaction region in the chamber.

The beams are then ad- justed for maximum transmission through the pinhole. The FIG. Because of this hole in the center. The beam path geometry described above allows maxi- mum flexibility for beam steering and adjustment of the in- In the setup of Fig. The setup exhibits are split before the EUV waveguide. The EUV output mode is imaged onto the center no active feedback on the beam pointing.

However, because of the detector using a multilayer mirror with a 1 m radius of the EUV and IR beams propagate along separate paths for curvature. The second beam path geometry shown in Fig. In this corresponding to a time step of 0. However, the smallest repeatable through while reflecting an annulus of the copropagating IR. After the optical delay, the This design is possible because of the difference in diver- IR beam is focused into the detector chamber using a 75 cm gence between the EUV and IR beams exiting the wave- or 1 m focal length lens placed before the pump-probe re- guide.

The EUV beam follows the same path as in the pre- combining mirror. Because of the hole in this mirror, which vious geometry and the waveguide output is imaged in the effectively acts as a reverse aperture, the mode of the IR interaction region. The IR propagates through a variable beam is modified upon reflection.

Figure 5 shows a calcula- length path and is also imaged to the center of the interaction tion that estimates this effect. In this calculation, the focal region using a 50 cm focal length lens.

The intro- ized translation stage, we use a closed-loop, piezoactuated, duction of side lobes contributes only slightly to the back- translator stacked on top of the motorized long travel stage. Changes in the time delay will also introduce changes in The EUV spot diameter in the setup is also estimated to the imaging condition of the IR. This beam etry exhibits interferometric stability in a controlled labora- spot size is consistent with the observed divergence of the tory environment. In Obtaining good spatial and temporal overlap between both geometries, the EUV mirrors were mounted in vacuum the IR and EUV beams is achieved by matching the IR beam on remotely controlled motorized mirror mounts to steer the with the residual IR light copropagating with the high order beam.

The skimmer also serves as barrier for differential pressure be- tween the gas jet chamber and the detector chamber.

As can be seen in Fig. This geo- metrical shape enhances the gas conductance and helps to FIG. The charged particles are guided to each side of the detector by a constant electric field generated by a series of copper electrodes, and by a jet chamber at a low pressure. This in turn helps the super- magnetic field generated by copper coils. The interaction region consists of sonic molecular beam formation. The skimmer is also cone a supersonic molecular gas jet directed upward, perpendicular to the direc- shaped to reduce possible backscattering that would be det- tion of the EUV and IR beams.

After the skimmer, the molecular beam still has some D. The interaction chamber divergence. The amount of divergence can be adjusted by varying the distance between the nozzle and the skimmer. A simplified schematic of the reaction microscope is For a distance of 13 mm between the nozzle and the skim- shown in Fig.

This translates collection efficiency of both electrons and ions, onto separate into an extended region of interaction along the beam propa- detectors. The time and position gation axis. The other two transverse dimensions are con- information makes it possible to fully reconstruct the mo- fined by the focused laser beam diameter. The longitudinal menta of the particles detected. This calculation requires dimension will introduce a slight uncertainty in the initial knowledge of the initial and final positions of the charged position of the molecular process that must be taken into particle.

It also requires constant and well-characterized elec- account in the analysis. Further improvements in chamber pressure, and reduc- tion in the size of the interaction region, may be possible A supersonic molecular gas jet supplies cooled mol- using a double-skimmer gas jet geometry. A cross section of the inter- The supersonic nature of the gas jet can be observed action region, the gas jet assembly, and the jet catcher are directly by the ion detector, and this provides a means to shown in Fig.

Figure 8 shows the x and the y positions of the ion impacts on the delay line detector that will be described later in the text.

Two main features are visible in Fig. The small offset region corresponds to ionization events in the gas jet. The offset comes from the large Py momentum component of the atoms or molecules from the jet. The centered line, in this case, is simply ioniza- tion of the background gas from the EUV beam propagating through the chamber. Following the work of Miller,46 the particle density can FIG.

The conical shape of the mer size, distance between nozzle and skimmer, stagnation chamber and of the area around the skimmer helps increase the pumping efficiency and keeps the pressure low in that region. For the D2 experiment presented in Sec. III assembly.

The gas jet chamber and the catcher were engineered to be as below, this calculation yields a particle density inside the close to the interaction region as possible. This gas jet local pres- bringing in the gas can be adjusted. Side to side adjustment ensures that the nozzle is centered on the skimmer. Up and down adjustment controls the gas sure is approximately 1x — 10x higher than the background density and the dispersion of the gas jet after the skimmer.

This characterization was done using O2 gas with 1. The long line across the center of the detection region arises from ionization of the background gas by the EUV beam propagating in the chamber.

The more localized and vertically offset intense region arises from the ionization of the gas from the jet, where the molecules have a high initial y momentum component. Guiding electric and magnetic fields In order to guide the charged particles generated in the interaction region, constant electric and magnetic fields are set up along the axis of the spectrometer.

The copper electrodes surround the detection region. The negative overall potential, com- FIG. The the spectrometer region will be excluded from the interaction off-diagonal feature shows an O2 dissociation channel. The opening is kept as small as possible to pre- vent distortion of the electric field. The precise lengths de- and the molecular gas jet velocity by looking at the dissocia- pend on laser beam pointing, and must be calibrated by using tion fragments of the double ionization of a molecule such as the detector.

The advan- trometer and between the copper electrodes and the drift re- tage of using a doubly ionized dissociation process is that the gion, are used to ensure that the electric field is constant products can be detected relatively background-free. How- throughout the spectrometer. In between the copper electrodes and the electron detector. The drift tube is approximately twice the length of the electron acceleration This dissociation channel is identified in the time-of-flight region, and its purpose is to temporally focus the electrons.

Finally, spot size. Ionization event occurs at T0. The charged particle FIG. The MCPs generate an electron shower that is defined as the distance from the center of detector. The nodes are charac- is collected by a delay line detector behind it. The holders for ratus. This self-calibration ability allows us to perform high the delay lines have a voltage of 80 and V for the ion resolution particle momentum detection, and reduces the side detector and the electron side detector, respectively.

The probability of systematic errors due to improper calibration. The electron zero determination as well. This current pulse travels to each side of each wire and electrons. They were constructed of ten gauge square copper the time of the signal is recorded and compared to a common wire with turns on each coil. In normal operation, ap- fixed trigger signal. This allows us to measure the position of proximately 3 A of current is supplied to these coils.

In order the particle impact. By varying the electric field the momentum at the time of interaction with the laser pulse during the data acquisition, multiple nodes can be observed.

If the TOF ron configuration. The third layer, time position of the node. The current through them is adjusted to ensure fraction discriminators. The constant-fraction discriminators that the nodes shown in Fig. The multihit dead time for the Downloaded 12 Jun to At one hit per channel, a TDC can acquire data at up to 25 kHz. If two ions hit the MCP, ten electrical signals will be generated to characterize them fully: two sig- nals from the two ends of the delay line corresponding to the FIG.

In order to detect all ten electrical signals asso- ciated with two ions in the same trigger event, they must the energy uncertainty for a cosine-square distribution along each be 20 ns apart. Alterna- charged particles inside the spectrometer. For ions, given the typically long cal when dealing with electrons. For an improperly set mag- TOF involved, the multihit dead time is usually not a signifi- netic field, the uncertainty in the electron energy, for a cant restriction for two-particle detection.

However, it can cosine-square distribution along the Pz axis, can become become important for electron detection. That is the reason a much greater than that along the Py axis. These uncertainty three layer delay line is used for electrons. The third layer values could be greatly improved by the implementation of a provides another set of redundant electrical signals that can double-skimmer geometry that could reduce the size of the be used for data reconstruction by the software.

Another important experimental consideration is the H. Experimental considerations count rate. In this article, we will discuss a source of energy However, the coincidence requirement does place a limit on uncertainty in the detected charged particles specific to our the count rate in order to match a detected ion with a de- setup: the extended interaction region.

As we described tected electron. This extended in- The three factors that determine the count rate are the teraction region, which results in an uncertainty in the initial flux in the EUV pulse, the gas density, and the molecular position of the molecular reaction, will result in an uncer- absorption cross section.

The multilayer mirrors select the tainty in the energy of the detected particles. After two reflections trons. If the cosine square is along the Pz axis, the uncer- event per pulse. For electrons, the situation is further tions peak in the 10 eV— keV range of photon energies49 is complicated by the presence of the magnetic field.

For a shown in Fig. It is not rare that, even for an overall event axis. This number fortunately becomes much smaller when count rate nearing the 2 kHz repetition rate of the laser, the looking at low energy electrons. For 1. The EUV pulse is represented by the long upward blue arrow. The detected electron energy associated with populating a particular state is shown by the downward orange arrows.

The IR ionization is represented by the short upward red arrow. While the laser channels. This can only be achieved by in- energy-ion energy correlation map. Here, the x axis corre- creasing the laser repetition rate. Increasing the pulse energy sponds to the kinetic energy of the detected electron, while might also increase the count rate, but the number of counts the y axis corresponds to the kinetic energy of the coincident per laser shot would become more than one and the coinci- detected ion.

Group 1 corresponds to photoelectrons with dence capability would be lost. A 20 kHz repetition-rate laser energies around 3 eV. This excited state is strongly dis- molecular cross section is low. This would make a 20 kHz system near ideal potential energy. Since the two fragments share energy equally in the kinetic energy release, III. This is exactly what we observe in group 1 shown in reaction microscope, we consider the excitation and dissocia- the correlation map of Fig.

Hence, this feature corre- tion of a D2 molecule. The photoioniza- out normal to the soft-x-ray polarization. However, since D2 is a relatively simple molecule, photon polarization are preferentially ionized. Again, en- evolution is represented in terms of potential energy curves. In a separate experiment,21 we performed such a study of the dissociation dynamics of shake-up state in N2. An IR pulse was then used to probe the nuclear dynamics as the molecule ex- ploded. The results were matched by a theoretical calculation indicating that a shake-up state of N2 was being probed.

This allows us to observe a molecular system as it evolves with femtosecond temporal and atomic-scale spatial resolution. Similar experiments are also being performed in various other molecules. The essential constituents of this sys- tem are femtosecond EUV pulses obtained from high har- monics, and the coincident three-dimensional momentum imaging method that serves as a reaction microscope.

This system has allowed us to perform the first femtosecond stud- FIG. The x axis shows the momentum component along the axis of the ies of molecular dynamics initiated by ionizing radiation. Future experiments will include exploring different mo- lecular systems, specifically O2 and CO, where the 43 eV photon energy is near the double ionization potential.

This Next, we address the third feature observed in Fig. As and IR radiation. This photoionization is accompanied by the launch of a probe different molecular states.

Finally, improvements in vibrational wave packet in the associated potential well. This work made at certain internuclear distances. However, it improves the time resolution capabilities and bandwidth se- Downloaded 12 Jun to The amplifier laser beam is focused into a 1 m long argon-filled hollow core fiber. The spectrum of the pulse is broadened through SPM.

The waveguide output is compressed in time by using a pair of chirped mirrors. The resulting 10— 12 fs pulse is focused into another hollow waveguide to FIG.

The output beam is analyzed by using after spectral broadening in the SPM waveguide and recompression with the an x-ray spectrometer and CCD camera. The harmonic emission is a 10— 12 fs pulse. The red curve corresponds to 50 torr of argon in the HHG waveguide with an intensity of 1. The har- monic emission is centered at the 27th harmonic.

The black curve corre- waveguide without the need for low gas pressure. We found sponds to 32 torr of argon in the HHG waveguide with an intensity of 1.

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