1：Experimental Design and Method Selection:

A Q-switch Nd:YAG laser was used to excite PMMA sample to produce the LIP. The laser was operated at 1064nm wavelength with a pulse width of 10 ns and a repetition rate of 10 Hz. The laser energy was adjusted by combining a half-wave plate with a GLAN laser polarizer in the laser beam path.

2：Sample Selection and Data Sources:

Polymethyl methacrylate (PMMA) was ablated to produce the plasma. The optical emission from the plasma was collected by a lens at a 45° angle to the laser beam and was converged into an optical fiber bundle.

3：List of Experimental Equipment and Materials:

Nd:YAG laser (Continuum, Surelite III), focusing lens with 100 mm focal length, spectrometer (Princeton Instruments, PI Acton, SP 500i), intensified charge coupled device (ICCD, 1024×1024 pixels, PI-MAX, Princeton Instruments), photodiode.

4：Experimental Procedures and Operational Workflow:

The pulsed laser was converged onto the target by a focusing lens. The focusing lens was placed on a computer-controlled translation stage to change the DTSFP. The optical emission from the plasma was collected and transmitted to a spectrometer. Each spectrum was an average of 20 laser pulses.

5：Data Analysis Methods:

The vibrational temperature for CN band system was determined by a least-square fitting program NMT applying the Nelder-Mead algorithm.