Electronic spectroscopy of carbon chain radicals using cw cavity ring down in conjunction with mass detection

The electronic absorption spectrum of the 2A'' − X 2A'' origin band of the
nonlinear carbon chain radical C6H4
+ was rotationally resolved by cw-CRD
spectroscopy [41]. It was analysed using a least-squares method and the rotational
constants of the ground and excited states were determined accurately. The 581 nm
band observed under the same discharge conditions is assigned to the same electronic
transition of C6H4
+ but involving the excitation of the ν12 vibrational mode in the
upper state based on comparison with ab initio results. The presented data provide a
basis for future observations of the C6H4
+ radical in both millimeter and infrared
regions.
A linear time-of-flight mass spectrometer was constructed to provide on-line
monitoring of the plasma discharge with a mass resolution of 1 amu at a range up to
120 amu. The results from the acetylene/helium plasma discharge are in good
agreement with those obtained using the reflectron TOF mass spectrometer and a
similar ion source [42]. To improve the experimental set-up, the following
modifications can be made:
• Transferring the signal from the oscilloscope directly to a PC via a
GPIB card will increase the speed of data processing;
• Computer control of the voltage applied will make the spectrometer
easier to operate;
• Using a metal grid at ground potential in front of MCP detector will
increase the flight time of ions improving the mass resolution;
• Installing a focusing lens will increase the number of ions arriving at
the detector, and therefore increase the signal on the oscilloscope.