A novel dual-frequency two-dimensional infrared instrument is designed and built that permits three-pulse heterodyned echo measurements of any cross-peak within a spectral range from 800 to 4000 cm(-1) to be performed in a fully automated fashion. The superior sensitivity of the instrument is achieved by a combination of spectral interferometry, phase cycling, and closed-loop phase stabilization accurate to ~70 as. The anharmonicity of smaller than 10(-4) cm(-1) was recorded for strong carbonyl stretching modes using 800 laser shot accumulations. The novel design of the phase stabilization scheme permits tuning polarizations of the mid-infrared (m-IR) pulses, thus supporting measurements of the angles between vibrational transition dipoles. The automatic frequency tuning is achieved by implementing beam direction stabilization schemes for each m-IR beam, providing better than 50 μrad beam stability, and novel scheme for setting the phase-matching geometry for the m-IR beams at the sample. The errors in the cross-peak amplitudes associated with imperfect phase matching conditions and alignment are found to be at the level of 20%. The instrument can be used by non-specialists in ultrafast spectroscopy.
A compact laser beam direction stabilization scheme is developed that provides the angular stability of better than 50 μrad over a wide range of frequencies from 800 to 4000 cm-1. The schematic is fully automated and features a single MCT quadrant detector. The schematic was tested to stabilize directions of the two IR beams used for dual-frequency two-dimensional infrared (2DIR) measurements and showed excellent results: automatic tuning of the beam direction allowed achieving the alignment quality within 10% of the optimal alignment obtained manually. The schematic can be easily implemented to any nonlinear spectroscopic measurements in the mid-IR spectral region.
In an effort to overcome the nonlinerities associated with a pneumatic system the variable structure control (VSC) technique has been employed to control the position of a pneumatic actuator. To accomplish this a model of a pneumatic actuator was developed to allow the control systems to be studied via simulations. The results from the simulations indicated that the VSC technique can position the actuator rapidly, via first-order trajectory, in response to a step input. An experimental study of the VSC control system was then performed. These experiments verified the simulation work, and demonstrated that inherent system delays and the quantisation of the control signals can cause and uncontrollable oscillatory behaviour; stability limits were established for three of the system parameters. It was also found, that the actuator could be placed within ±5 mm of the desired position.
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