The imaginary component of the optical constants, k, for a water sample at 120 K (black line). Naturally occurring HDO appears as the broad band near 2420 cm⁻¹ (4.13 μm). The interpolated baseline across the HDO band (dashed red line). The best HDO band subtraction and resulting deuterium-free effective optical constants for water (blue line) using Equations (8) and (9), corresponding to D/(H+D) = (1.58 ± 0.05) × 10⁻⁴. Our deuterium abundance is in good agreement with the terrestrial value, which increases our confidence in our procedures and in the resulting HDO effective optical constants.