Simulated Spectrum

The option of displaying a simulated spectrum serves comparing the spectrum predicted from molecular parameters with the experimental spectrum which is supposed to be assigned with the LWW program. In the general setup the LWW program contains the predicted transition wavenumbers (WnCalc displayed as numerical entries e.g. in the Assignments window) calculated from the Energy file. When the molecular parameters used for such prediction are close to the real ones, the experimental wavenumbers may lie so close to the corresponding predictions that the closest peak found by the program and displayed as WnObs in the Assignments window is the one that should be assigned to the quantum numbers corresponding to WnCalc. This is, however, only a rare case, especially in the first stages of assignments. At this level of course, the Loomis-Wood diagrams offer a much more efficient visual representation of the relationship between the predicted and obseved spectrum. With larger discrepancy between the real molecular parameters and those used for predictions the efficiency of the Loomis-Wood representation weakens and it becomes more useful to search visually for similarities between the experimental spectrum and the predicted one (or its individual parts) over the whole bands(s).

The predicted spectrum can be displayed as a second trace of the Spectrum window, below the experimental one, by the Simulated spectrum command in the Spectrum Menu. This requires to generate first a 'catalog' file which contains for each spectral line besides the set of all quantum numbers and transition frequency the relative intensitie of the line. The catalog file can be generated in the simplest approximation of an unperturbed band with relative intensities derived from Honl-London factors directly from the Energy file by the corresponding option From Energy File of the Simulated spectrum command. This command opens a dialog window (Simulated Spectrum from Energy File) whose logic follows closely the Define Series of Branches option of the Add New Branch window. The set of branches defined in this dialog window can be subsequently calculated and stored as a catalog file in the project directory by clicking on the Create and Save Catalog File command button. When the catalog file of the band needs to be composed in several steps, the second and further series can be appended to an already existing file by saving with the Add to Existing File switch selected. The simulated spectrum from the final catalog file is then displayed by the From Catalog File option of the Simulated spectrum command.

This intermediate step of generating the catalog file enables for example having different parts of the band(s) stored in separate files. In additon to this, the catalog file may be as well generated by an external program which can incorporate calculations of intensities perturbed by vibration-rotation interactions. The external program either has to generate the catalog file in the internal format used by the LWW program, or the user can define an import template for the catalog file by the Project Options command of the File Menu.

The selection of the catalog file for simulation of the spectrum is done in the dialog Simulated spectrum window which is opened either right from the dialog for generating the catalog file or by the From Catalog File option of the Simulated spectrum command. The simulated spectrum is displayed by clicking on the Make Spectrum command button. Three characteristic features of the simulated spectrum can be changed.

• The Peak Width (FWHM) is controlled by the value of the corresponding edit box with up/down control. The edit box has one predefined value (0.0020 cm^-1 and default step (0.0005 cm^-1) but can be as well edited to any desired value. This item can be edited before displaying the simulated spectrum. Later the peak width can be modified again by setting a new value and clicking again on the Make Spectrum command button.
• The Change Intensities command has the assocaited intensity scaling factor in the adjacent edit box. This factor is applied with every click on the Change Intensities command button.
• The Shift spectrum command works in an analogous way. The cumulated shift, which is displayed in the text box below, can be reset to zero by clicking on the Undo Shift command button.

There are three levels of filtering (selection of peaks) from the catalog to the displayed simulated spectrum, file controlled by the All Trans / Branches / Sim-tag. All transitons contained in the selected catalog file are displayed when the default filter All Trans is selected. When the Branches filter is selected, only the branches from the Branches window table are filtered out from the catalog file and displayed. The selection of Simulation tag (Sim-tag) limits the set of displaeyd lines only to branches with a Sim-tag in the Branches table set to 1. This tag should not be confused with the Exp-tag in the same table, which serves as a filter for exporting assigned transitions and displaying peak markers for predicted/assigned transitions in the Spectrum window. Changing both the Exp-tag and Sim-tag is done from context menu invoked by a right click on any line in the Branches window or on a set of lines of this table (highlighted by dragging the mouse cursor over these lines with the left mouse button held down).