PGOPHER is a general purpose program for simulating and fitting rotational, vibrational and electronic spectra. It represents a distillation of the experience of the Bristol laser group in using spectra for many different purposes. PGOPHER will handle linear molecules and symmetric and asymmetric tops, including effects due to unpaired electrons and nuclear spin, with a separate mode for vibrational structure. The program can handle many sorts of transitions, including Raman, multiphoton and forbidden transitions. It can simulate multiple species and states simultaneously, including special effects such as perturbations and state dependent predissociation. Fitting can be to line positions, intensities or band contours.
PGOPHER is designed to be easy to use; it employs a standard graphical user interface and the program is currently in use for undergraduate practicals and workshops as well as research work. It has features to make comparison with, and fitting to, spectra from various sources easy. In addition to overlaying numerical spectra it is also possible to overlay pictures from pdf files and even plate spectra to assist in checking that published constants are being used correctly.
The program is freely downloadable from the dedicated web site (http://pgopher.chm.bris.ac.uk) for Microsoft Windows and Linux, with a beta version available for the Apple Mac. The program is released as open source, and can be compiled with open source tools.
The figure above shows an example of a screenshot from PGOPHER – in this case a simulation (lower trace) of the well known IR absorption of HCl. Note the isotopic splitting between H35Cl (in blue) and H37Cl (in green). The upper trace is a synthetic spectrum derived from the HITRAN database for comparison.
A more advanced simulation is shown above in which rotational band contours are analysed for two overlapping vibronic transitions of the NCN free radical in a flame. Simulated bands in blue and green combine to give the full simulated spectrum in black. This simulation is compared to an experimental spectrum shown in red. This analysis forms part of a collaboration with colleagues at the University of Lille.
Another application of PGOPHER is illustrated above, this time to determine the product state distribution following a reaction. In the example shown, ammonia molecules are formed by the photodissociation of ammonia dimers and trimers. The upper trace is the experimental spectrum, and the lower trace is the simulation, involving two different excited vibrational states of ammonia in the ground electronic state. One conclusion from this study, which was performed in collaboration with the group of Fleming Crim at the University of Wisconsin, was that the binding energy of the trimer was in the range 1700 to 1800 cm-1.
Re-investigation of the spectroscopy of the A 3Πu– X 3Σ–g transition of the NCN radical at high temperature: Application to quantitative NCN measurement in flames, N. Lamoureux, C.M. Western, X. Mercier, and P. Desgroux, Combustion and Flame. 160, 755-765 (2013).
Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters. A.S. Case, C.G. Heid, C.M. Western and F. F. Crim. J. Chem. Phys. 136, 124310 (2012).