Poor detection limits of two-photon excited fluorescence in cylindrical capillaries are attributed to photothermal expansion and beam astigmatism. Photothermal expansion is demonstrated for excitation in a 1 cm cell and is inferred for the larger diameter capillaries. Astigmatism is caused by focal differences between rays in a plane longitudinal to the capillary and rays in a plane transverse to the capillary. Data were obtained by integrating the fluorescence from variously sized cylindrical and square capillaries, and by photographing the fluorescence within a 1 cm cell.
K. S. Overway and F. E. Lytle, "Two-Photon Excitation Profiles in Cylindrical Capillaries," Appl. Spectrosc. 52, 928-932 (1998)