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Neutral Cesium has thermal lines extending from the near ultra-violet, far down into the infra-red. Strong lines occur from 3876 to 34900 angstorms. There isn't really a lower limit (in energy). You can go as far down into the IR as you want by jumping electrons to higher and higher orbitals. It's just that the lines stop being very observable after a while. Single Ionized cesium has lines in the vacuum ultraviolet starting at 591 Angstroms. The following are some of the prominant lines for neutral cesium
Wavelength Relative Angstroms Intensity 3876.15 2100 These are in the near ultraviolet 3888.61 600 4555.28 1000 The famous blue lines, used for 4593.17 450 spectroscopic identification 5465.94 60 A bunch of relatively weak 5502.88 37 greenish lines. 5635.21 100 5664.02 210 5745.72 27 5838.83 59 A couple of yellow lines 5845.14 300 6010.49 640 Lines in the orange 6034.09 86 6213.10 1000 6217.60 170 6354.55 320 6586.51 490 A couple of strong and 6628.66 97 some weaker red lines 6723.28 3300 6824.65 200 6870.45 300 6973.30 4800 6983.49 980 . . . And on down through the infrared . . . 8521.13 59000 Two exceedingly strong ones, 8943.47 61000 used for spectrascopic ID . . .
Note that this spectrum looks like it would appear be mostly red to the eye, given the number of strong lines around red, whereas the actual appearance of burning cesium or flames colored with it tends from blue towards lavender and reddish violet. The explanation is that there are a number of singly ionized lines that also figure in, depending on the temperature of the flame. The strongest of these, at 4603.79, plus other blue and green lines add significantly to the bluish appearance. Some prominant lines from the singly ionized spectrum follow. Note that the relative intensities don't necessarily compare directly with those for the neutral atoms.
Wavelength Relative Angstroms Intensity . . . Lots of stuff up in the UV . . . 4039.85 8000 Violet line 4264.70 14000 Strong blue lines 4277.13 18000 4501.55 12000 4526.74 20000 4603.79 100000 4830.19 25000 4870.04 19000 4952.85 37000 Strong green lines 5043.80 27000 5227.04 75000 5249.38 29000 5274.05 11000 5349.13 10000 5370.99 22000 5563.02 39000 5831.14 24000 A yellow line 5925.63 51000 A couple of orange ones 6128.61 9800 6495.53 8300 And some red ones 6536.44 10000 6955.50 37000 6979.67 16000 . . . And down through the IR . . .