text: In liquids, three MTV approaches have been classified: MTV by direct phosphorescence (using a phosphorescent dye), absorbance (using a photochromic dye), and photoproduct fluorescence (typically using a caged dye).
  MTV based on direct phosphorescence is the easiest technique to implement because a single laser is needed to produce a luminescent excited molecular state. The phosphorescence signal is generally weaker and harder to detect than fluorescence.
  The second technique called MTV by absorbance relies on the reversible alteration of the fluorescence properties of a photochromic dye. The scheme showed good results in alcohol and oils, but not in water in which typical dyes are not soluble.
  The third variant of MTV was first deployed in liquids in 1995 under the name "photoactivated nonintrusive tracking of molecular motion" (PHANTOMM). The PHANTOMM technique initially relied on a fluorescein-based caged dye excited by a blue laser. More recently, a rhodamine-based caged dye was successfully used with pulsed UV and green lasers.