*4.1. Layout of the NIR/MIR Four-Color Laser Absorption Instrument*

Figure 3 shows the schematic of the instrument consisting of the sending, auxiliary, receiving, and free-beam guiding units. The sending, receiving, and free-beam guiding units are enclosed in acrylic glass or metal boxes and purged with dry nitrogen to protect all optics from dust deposition and avoid spurious absorption from water vapor and CO2 outside the probe region. Indium trifluoride (InF3) single-mode MIR-transmitting optical fibers (Thorlabs, P3-32F-FC-1) are used to deliver the laser light from the sending unit to the free-beam guiding unit composed of optical windows and purged lens tube components with a varying diameter (Thorlabs lens tubes, 1" and 1/2" diameters). The attenuation of the InF3 fiber in the MIR range is <0.35 dB/m and <0.52 dB/m in the NIR range. However, it should be noted that fiber for NIR radiation does not behave as a single-mode fiber, which causes limitations in the ability to focus the beam and the quality of the beam profile, but these effects do not cause any crucial limitation for the application here.

The laser beams leaving the sending unit (see below) are collimated by an off-axis paraboloid (Thorlabs, RC02APC-P01, Bergkirchen, Germany) mounted on an *xy* translation stage fixed at the free-beam-guiding lens tube. Inside the lens tube, the light beams are steered with a gold-coated mirror and split into two parts by an optically polished CaF2 wedge (3 degrees, Korth Kristalle GmbH, Altenholz, Germany). The reflected part is transmitted through a wedged window (1◦) towards the auxiliary unit. The transmitted laser beams are guided via 1/2"-diameter lens tubes through the flame and then towards the receiver unit. Each lens tube is closed with wedged (0.5◦) CaF2 windows (Thorlabs, WW50530) and apertures (Thorlabs, SM05D5D) and flushed with dry nitrogen to avoid absorption by room air water vapor. The flat flame is operated using a McKenna burner (Holthuis & Associates, Sebastopol, CA, USA) mounted on a lab jack (Thorlabs, L490/M) for height adjustment.

**Figure 3.** Optical layout of MIR/NIR laser absorption instrument.

All detector outputs in the sending and receiving units are acquired by two DAQ cards (PXIe-6124, National Instruments, Austin, TX, USA) in the PXI system, which also worked as a function generator for the current modulation of the lasers. The system also controls and remotely operates stepper motors for mechanical translation stages.
