Beneficios Perinatales

Infrared laser spectroscopy is an ideal method for probing both the structure and dynamics of molecular dopants embedded in helium droplets and the dynamics of the droplet, using the molecule as a handle. The infrared spectra of smaller molecular systems are often rotationally resolved due to the weakly interacting and superuid nature of the solvent. As discussed in the previous chapter, the long rotational coherence times are a manifestation of the lack of 'bulk' droplet excitations at the energies of the excited rotational states. At the temperature of the droplet (0.37 K), the rotational contours only span about 1 cm−1_{, and this width is often smaller than the shifts that result}

from complexation of one molecule with another. As a result multiple isomers and/or oligomers are easily resolved [80] in the spectral region accessible to our lasers, namely the H-X stretching region of the infrared (2-4 µm). Vibrational excitation of dopant molecules leads to the evaporation of several hundred helium atoms. Again, in most cases6_{, putting internal energy into the system leads to the creation of droplet excitations,}

which eventually results in evaporation, at least on the timescale of the experiment (∼1

ms). Assuming each atom removes 5 cm−1 _{of energy [25], a 3000 cm}−1 _{excited vibrational}

state will lead to the evaporation of 600 helium atoms. It is this reduction of the on- axis beam ux that allows us to acquire an infrared spectrum of the dopant molecule. Using phase-sensitive techniques (mechanically chopping the infrared laser), a thermal bolometer detector measures, as a function of laser frequency, the dierence in the beam

energy with and without the laser. The various lasers and laser techniques applied in each experiment will be described in more detail in the following chapters, and we focus here on the general aspects of the laser induced depletion technique.

After the droplets have been doped in the second stage of the apparatus, they pass into the third dierentially pumped stage containing the laser multipass (MP) cell [81], shown schematically in Figure 2.6. The MP cell is simply two parallel gold mirrors aligned

Figure 2.6: Schematic diagram of the linear laser multipass cell equipped with Stark plates.

such that the laser beam will intersect the droplet beam 50-100 times. The laser beam is focused into the MP cell with a 2 meter / 1 meter telescope, and we easily achieve a factor of 50 improvement in the sensitivity in comparison to a single pass alignment. For the two infrared laser double resonance experiments, spherical mirror MP cells were used (see Figure 2.5). In the spherical MP arrangement, the laser beam enters at a slight angle through a small hole in the top mirror. A 15 cm short focal length lens focuses the light to the center of the MP cell, and the spherical mirrors refocus the light for every pass. This arrangement has the advantage of producing a small photolysis volume, and with two spherical mirror MP cells, we obtain a well dened spatial separation between the upstream pump and the downstream probe.

Vibrational excitation followed by relaxation and helium evaporation results in he- lium atoms scattered to a mean lab-frame angle of≈10◦ [6]. As a result, the evaporated

helium atoms largely miss the downstream bolometer detector. As described above, the vibrational relaxation induced depletion of the beam is recorded as a function of infrared laser frequency. The thermal detector used to monitor this depletion is a doped silicon bolometer (Infrared Laboratories) mounted to the copper baseplate of a liquid helium dewar (see Figure 2.6). A thin 5 mm x 2 mm (long axis vertical in our apparatus) dia- mond collector is mounted to the surface of the bolometer to enhance the active detector surface area. The vapor pressure above the liquid helium in the dewar is lowered with a

V_bias

beam

signal

Figure 2.7: Schematic diagram of the bolometer circuit. The bias source is a 15 V mercury battery and the reference load resistor is 20 MΩ.

small mechanical pump, reducing the operating temperature of the bolometer to 1.6 K. Since the liquid is superuid at this temperature, the bolometer is connected to a very stable thermal bath. The lowered temperature and increased stability reduces the inher- ent Johnson noise (thermal uctuations), along with lowering the heat capacity of the doped Si bolometer. The latter consequence is particularly important since we measure the electrical response of the bolometer due to a temperature change. By lowering the temperature to 1.6 K, the same power input results in a larger bolometer temperature rise.

A schematic of the bolometer circuit is given in Figure 2.7. The bolometer is biased with a stable 15 Volt mercury battery, and a 20 MΩ load resistor is chosen to be much

larger than the resistance of the bolometer, resulting in a constant current. With a con- stant current, small deviations in the bolometer's resistance associated with temperature changes can be converted to voltage changes that are amplied and easily measured with a lock-in amplier and phase sensitive techniques. First stage amplication is accom- plished with a small JFET transistor, which is mounted onto the cold stage next to the bolometer. The JFET location was chosen to reduce the length of the wires from the detector to the amplier, hence reducing the microphonic noise originating from the mechanical vibrations of the nozzle coldhead. The responsivity of the bolometer as mea- sured in volts of output per Watts of input is S = 7.2 x 105 _{V/W, with a background}

(Johnson limit) noise of about 50-100 nV/√Hz. This leads to a sensitivity on the or-

der 10−13 W/√Hz (noise equivalent power). From a simple Newtons law back of the

envelope calculation, a droplet beam with 1012 N = 3000 droplets/s traveling at 450

m/s will result in 1.4 Volts of signal. However, only one in 102 droplets hit the detector

element due to the natural divergence of the beam along with the ∼ 80 cm long path

between the nozzle and the detector. As a result, we would expect 14 mV of droplet beam signal, but in practice it is 5 times lower than this value. Unfortunately, much of the droplet beam kinetic energy is wasted as it is used to evaporate and/or fragment the droplet upon impact with the detector. For anN = 3000helium atom droplet, assuming

the binding energy is pair-wise additive (ie. 5 cm−1 _{per helium atom [25]), we estimate}

that as much as 30 nW of power is lost, corresponding to ∼ 2 mV of beam signal. In

general, we nd that for the C-H stretch R(0) line of HCN (I ' 120 km/mol), 5 mW

from the F-Center laser focused into the linear MP cell leads to about 1.0-1.5 % depletion of the beam signal, corresponding to 28 to 42 µV of laser induced depletion signal, or a signal to noise ratio of 560:1 to 840:1. However, the transition intensity of heavier molecules is spread out over several thermally populated rotational states, making the

sensitivity correspondingly less. Nevertheless, as described in the next section, the search for weak transitions can be greatly facilitated with the pendular state technique, which collapses the rotational structure into a single peak centered near the band origin (i.e. for transitions having transition moments parallel to the laser electric eld).