Pulsed Tunable UV Laser

　

        Two-color resonance multiphoton ionization and mass-analyzed threshold ionization spectroscopy experiments require two adjustable pulsed UV lasers. The first UV laser is used as an excitation light source to excite molecules from the ground state (S0 state) to the first electronically excited state (S1 state). The second UV laser is used as the ionization light source, which takes the molecules in the S1 state to the ionic state.

　

1. Pulse IR laser

    The first part is a solid-state Nd:YAG laser. When the Nd:YAG rod (active medium containing Nd3+ ions) absorbs energy from a flash (xenon) lamp, it emits IR laser with a wavelength of 1064 nm. Using a set of Q-switch, the IR laser can be released instantly after accumulating to high energy. Therefore, the time delay control of the flash lamp and the Q-switch is an important factor to determine the laser pulse energy. Based on our operational experience, the maximum IR laser pulse energy can be obtained when the and the delay time is set about 235 ms.

    The output IR laser with a wavelength of 1064 nm can be converted into VIS or UV lasers with wavelengths of 532, 355 and 266 nm after frequency multiplied by harmonic generators (HG). Taking generation of VIS laser of 532 nm as an example, when the 1064 nm laser passes through the second type (Type II) KD*P (potassium dideuterium phosphate) crystal with a specific incident angle, a portion of the 1064 nm laser is converted into 532 nm laser. The frequency-doubled 532 nm VIS laser and the 1064 nm IR laser emitted from the HG at the same time. A set of dichroic beamsplitters (Dichroic Beamsplitters) with an incident angle of 45° must be used to separate the 532 nm VIS laser and the 1064 nm IR laser. The dichroic beamsplitter is coated with a layer of selective reflection material, which reflects light of a specific wavelength range (including 532nm), and light of other wavelengths (including 1064nm) passes through the lens. What is separated out is pure laser light of a specific wavelength (532 nm), which is used as the excitation light source for the dye laser. The NdYAG laser used in our early experiments was Quanta-Ray GCR-3 (the pulse frequency is 10 Hz, and the pulse energy is 1010 mJ/pulse @ 1064 nm ; 510 mJ/pulse @ 532 nm; 305 mJ/pulse @ 355 nm; 150 mJ/pulse @ 266 nm).  In our early studies on molecular clusters, cluster ions were generated through the non-resonant multiphoton ionization process. The UV lasers with a wavelength of 355 nm and 266 nm were commonly used. Later, in the resonance-enhanced multiphoton excitation and ionization experiments, the 532 nm laser was commonly used as the excitation light source for the dye laser.  

　

2. Dye laser

　

A. Basic principles

    A dye laser is a passive laser, which requires a pump (excitation) light source (such as 532 nm laser). The active laser medium is a dye dissolved in a liquid solvent (such as methanol). The basic principle of laser generation is briefly described here with Rhodamine 6G (also known as Rhodamine 590) dye. Molecular energy levels S0, S1, and S2 are the ground state, the first and second electron singlet excited states, respectively, and the molecular spin is anti-parallel in the excited state. T1 and T2 are triplet states, and the electronic spin of the molecule is parallel, because the transition between different spin states is forbidden, and the transition from singlet to triplet is less than singlet-single state or triplet-triplet state occur. Each electronic singlet state and triplet state has a vibration-rotational energy level with a small energy gap (100-200 cm-1), and each vibrational energy level has a smaller energy gap (<1 cm-1) the rotational energy level. Such a dense vibration-rotational energy level produces very broad spectral lines due to the disturbance of electronic states and the collision of solvent molecules. Such a wide spectral line makes the emission and absorption spectra of dye molecules continuous, so it becomes an ideal substance for tunable wavelength emission.

    Absorbing a photon (e.g. 532 nm) from the excitation laser, the dye molecule in the S0 state is excited to a certain vibration-rotation energy level in the S1 state, and is collided by solvent molecules in a higher excited state. After a few nanoseconds, the dye molecule emits a photon to release excess energy. This radiation transition is called fluorescence.  Most molecules will decay from the excited state S1 to the ground state S0 through this process. Crossing between systems (from singlet state S1 to triplet state T1) may also take place, taking on the order of several ms (microseconds). The radiative transition from T1 to the ground state S0 (phosphorescence emission) is forbidden, and its lifetime is between ms and ms (milliseconds), so the triplet state T1 seems to catch the excimer, making the dye less efficient. Further reducing dye efficiency in the triplet state is the spin-allowed transition from T1 to T2 or higher triplet states. These processes absorb the excitation laser light and reduce the efficiency of fluorescence generation.


B. Structure of dye laser

      The dye laser used in our early experiments is Quanta-Ray PDL-3, which provides adjustable laser ranging from 380 to 960nm, through the interface of MCI-2 and drives the grating stepping motor The function allows us to remotely control the output wavelength of the dye laser with computers and automated programs.

The standard structure of the dye laser includes optical components such as light splitting, focus, oscillation, amplification, and grating, and a dye circulation system. When the 532 nm laser beam is introduced, a small part (about 10%) of the pump (excitation) source is focused by a cylindrical focusing lens (cylindrical lens), it is used to excite the dye molecules in the oscillator (oscillator) dye dish, and the rest (about 90%) of the excitation beam is delayed for a period of time (about 3 ns) and then excited to amplify (amplifier) dye molecules in the dye dish. The oscillator part still contains a precision monochromator combined with a gain medium, output lens (output lens), six-prism expender and a 4-inch long grating. This design provides high efficiency, wider tunable range, and narrower output light wavelength for systems using only one grating. The grating has the function of splitting light, and the light emitted by the excitation contains a range of wavelengths, and the divergent beam of the six-prism expender shines on the grooves on the surface of the grating. Only the light of a specific wavelength is collected and reflected in a specific optical path. In the oscillation cavity After oscillation, due to the enhanced interference of the light waves, the amplitude of the light waves is increased, and the energy is increased to the point where it can pass through the output lens to complete the effect of stimulated amplification. The output beam overlaps with the excited beam of the amplifier through a telescope lens group to increase the output gain. Therefore, the wavelength width of the dye laser output light is closely related to the angle of the grating. By using different dyes, the number of grating stages and the angle of the grating can be set through manual or computer control of the stepping motor, and lasers with specific wavelengths ranging from 380 nm to 960 nm can be obtained. The output power of the dye laser is closely related to the touch excitation light source, the dye molecule itself, the nature and concentration of the solvent. After exciting the dye with 532 nm as the wavelength of the touch excitation light source, adjust the grating at different angles to measure the power of the output light (pulse energy), if the output power is plotted against the wavelength, a dye emission (or absorption) efficiency curve can be obtained.

　

C. Wavelength frequency multiplier

       Because the research topic requires tunable pulsed UV light, the light output by the dye laser must be frequency doubled. The original manufacturer of the wavelength extender used in this experiment is Spectra-Physics, and the product model is Quanta-Ray WEX-2. Appropriate selection of KD*P crystals with different angles can produce UV light with adjustable wavelengths ranging from 432 nm to 216 nm. For example, the output range can be frequency doubled from 310-440 nm by using a C1 crystal with a cut angle of 50°, and a cut angle of 58°. The output range of the C2 crystal after frequency doubling is from 285-350nm, and the output range of the C3 crystal with 74°cut angle is from 267-290nm. Therefore, when doing experiments, we must pay attention to the scanning range to select the appropriate crystal.

       The visible light output from the dye laser is reflected by the beam combiner of WEX-2 to the KD*P crystal with frequency doubling function. The refractive index of incident visible light of different wavelengths is different, so the phase-matching of the crystal needs to be fine-tuned KD*P crystal angles within the group to get the most powerful UV light. The principle of phase-matching of the fine-tuning crystal is as follows: part of the output ultraviolet light is irradiated on the two-quadrant photodiode. If the beam is deflected, the bridge on the irradiated diode circuit will generate a current, prompting WEX-2 Automatically fine-tune the angle of the crystal to balance the current on both sides of the diode until no current passes through the bridge, so that the maximum output power of ultraviolet light can be obtained.

      Since the conversion efficiency of wavelength doubling is usually only about 10%, the light source after wavelength doubling is mixed with the original visible light and the doubled ultraviolet light. Therefore, we must use Pellin Broca to separate visible light and ultraviolet light. According to the requirements of experimental conditions, the intensity of UV light can be controlled by Neutral Density Filters with different penetration rates. After the intensity is focused by a cylindrical focusing lens, it is introduced into a high vacuum chamber and intersects with molecular beams, as a light source for exciting or dissociating molecules.

　

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